In the original Highlander Ramirez says this to MacLeod:
You are safe only on Holy Ground. None of us will violate that law. It's tradition.
Even Kurgan obeyed it, despite his lust to have the prize:
Holy ground, Highlander! Remember what Ramirez taught you.
Were there Immortals who did not abide by this tradition/law (in the movies or in the TV series) intentionally?
This answer, shows that at least in the extended canon of the books Duncan was apparently tricked into violating it.
The whole Holy Ground thing is complicated.
TL;DR:
Jacob Kell from Highlander 4: Endgame claimed to not care about Holy Ground; and violated one interpretation of the rules.
Connor MacLeod and Kane fight on what is Holy Ground in Highlander 3
According to Wikipedia:
The rules of the Highlander universe state that Immortal combat is strictly forbidden on Holy Ground, though in Endgame (H4) Jacob Kell beheaded multiple immortals and there were no repercussions, despite them being on holy ground.
Interestingly enough, due to fan pushback,
Criticism of the holy ground beheadings [ in Endgame ]resulted in the removal, from the DVD edition, of all references describing the Sanctuary as "holy ground".
In Highlander II: The Quickening General Katana states that the 'Golden Rule' is that Immortals must not fight on Holy Ground. This would explain Jacob Kell's actions in Highlander: Endgame when he beheads several Immortals at The Sanctuary, a location that Immortal Methos referred to as Holy Ground. The Immortals at the Sanctuary are all in metal restraints meaning that Immortal combat did not take place per se. The idea that they were killed on Holy Ground, however, was still controversial enough with fans that subsequent releases of the film removed any reference to The Sanctuary being on Holy Ground.
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The theatrical trailer of Highlander: Endgame features Kell stating that he does not care about The Game. However, when confronted by Connor MacLeod in a cemetery, Kell decides to step into the road running alongside it before fighting him. Connor at any rate, holds his sword to Kell's neck but stops short of actually beheading him.
However, there is an independent canon confirmation (Wiki again)
At least a hint of those unknown consequences can be glimpsed in Highlander 3 when despite the rules Kane engages Connor in battle on the grounds of a sanctuary. A foreboding atmospheric effect begins with a closeup on a statuette of The Buddha (the location on which they are fighting is a former Buddhist shrine). The fight culminates with the destruction of Connor's sword which shatters into thousands of fragments. The two immortals wisely choose to postpone their battle.
Also,
According to Joe Dawson there is a Watcher legend about "two Immortals going at it in a Temple of Apollo" in 79 A.D. in Pompeii, which may have led to the eruption of Vesuvius. However, he admits that this is only a rumor and no one knows if it is true or not.
There is an interesting side note that seems to indicate that Duncan MacLeod refrained from killing even a mortal on Holy Ground (specifically, Horton):
... in Season 2 episode 'Unholy Alliance' part 1, Horton meets MacLeod on holy ground and claims holy ground protection when he sees that MacLeod wants to attack him. And although Macleod disarms Horton in the graveyard in Season 2 episode 'Counterfeit' part 2, he waits until Horton has run out the gate before he confronts him.
In Highlander III, Kane attacks Connor McLeod inside a buddhist temple. Connor warns him about Holy Ground, but Kane ignores it and attacks nonetheless. His strike shatters Connor's Masamune katana but, before he can deal the killing blow, thunder and lightning erupts in the temple, similar to a Quickening. Kane then leaves without killing Connor.
As far as intentions go, Kane's actions were pretty intentional. But, as you can see, there seem to be measures taken to enforce the law regarding respect of the Holy Grounds. And in Highlander III, at least, those measures were enough to scare off an Immortal and prevent him from killing Connor.
The Highlander universe is most controversial of most franchises made. Like Highlander 2 challenges what has happened in part 4 Endgame, Connor's death, and TV-series completely reverses part 1 and 3 - as Connor is a victor of the prize.
Also worth mentioning that according to head kills - it might turned out that there are millions of Immortals out there fighting all the time or they just go to hunting season once in a century for a few years or so. Don't forget that there were "hunters" Immortals who apparently kept killing all the time until were stopped.
Seems that most heads Duncan took (190 was Jacob Kell) was in a short period of 90s. Same time he questioned power of Methos in a fight against Kallas, since he was "out of the game" for too long.
As I see it, Highlander facts not needed to be taken straightly but overall-y. Each story might happen or might not - if it controversial to other.
As for question. Best option to pick is, that an Immortal can't assault another Immortal with attempt to murder him on Holy Ground, with exception if your evilness had given you special ability with Quickenings token. Like Kell obviously did.
What has confused me for a long time was in season 4, in "One minute To Midnight" and "Judgement Day", when Joe is set up & kidnapped to be brought before the Watchers to face the charges of breaking his oath and falsifying chronicles. Joe's friend and the head of the meeting, Jack Shapiro, brings to light the fact of more Watchers have died since Joe and Duncan became friends.
MacLeod comes to bust Joe out and ends up putting his own head on the line to testify with Joe. Then the death knell sounds as Dawson is found guilty when Jack's son, a new Watcher in the field is killed. MacLeod tries to escape with him but Joe refuses to leave the Watchers' headquarters.
The next morning Joe is set to be executed. Just before Joe is shot, him and several of his fellow watchers are mowed down with machine gun fire. MacLeod is on his way to make another attempt to save Dawson and passes by the gunman which Mac senses to be an Immortal. Mac finds Joe barely alive and seeks refuge at the Watchers' bookstore. Jack Shapiro believes Duncan MacLeod was responsible for the massacre at the headquarters. Jack then orders the Watchers to hunt MacLeod and kill him on sight.
MacLeod tries to infiltrate where Shapiro has established the new interim headquarters but is shocked when he finally meets the Immortal that has gotten him blamed for all the Watcher deaths. It is his long time gypsy brother, Jakob Galati. MacLeod rushes to catch up with him but is shot through the lung by a Watcher, which then quickly closes proximity to deal the final strike.
Just as the blade is about to drop, the Watcher is killed by Jakob and Mac 'dies'. Duncan wakes up to Jakob violently playing guitar, and were it not for the chaos mixed within both men, it would almost seem like old times. Duncan finally learns the horror of why Jakob is there. After Jakob and his long time Immortal love Irana had settled down, they were pursued by men who burned their home. Jakob had to watch as notorious rogue Watcher & Macleod's long time nemesis, James Horton beheaded Irana, and Jakob helplessly took her quickening.
MacLeod realizes he is caught in a seemingly no-win situation, seeks to end the war by setting up a meeting between Dawson and Jakob in hopes of some beginning to resolve. Unfortunately, Dawson set up Jakob to be brought before Jack Shapiro, and Jack's only idea for resolve is revenge with the blade for Jakob. Macleod learns from Methos the Watchers are holed up at a mortuary.
MacLeod stalks his way into the sanctuary and learns the diabolical twist of intentions. Jakob is taken to a side room to await execution by Jack as both MacLeod and Joe plead for understanding. Jack sets goons on MacLeod long enough to delay him and a flash of the quickening sends MaclLeod flying signifying Shapiro's revenge, the death of a friend trying to avenge his love, and mistrust abound.
To my understanding, the Watchers set up the hideout at the mortuary because it WAS HOLY GROUND. I believe it was a mortuary in a cemetery. Nonetheless, it is worth watching again.
There was one episode (Highlander S5E08, "Little Tin God") in which Duncan's opponent was an Immortal who in the past had spent centuries acting as the god of a Mesoamerican tribe (they were awed because they could keep sacrificing him over and over...); shortly after Duncan and another man found the tribe, everyone but the Immortal died of some Western disease (maybe smallpox?), and so the Immortal swore vengeance against Duncan.
He carried out his vengeance in the present by killing teenage pre-Immortals, then when they resurrected he convinced them that HE had resurrected them and so was their god. He sent several of them to attack Duncan, and because they weren't properly educated, they tried to attack him on Holy Ground (which rather threw him for a few seconds and nearly cost him his life).
