TL;DR: The point wasn't to crush people's souls or even the score, it was to make Rick look powerless in front of his own people. The deaths the group has already endured are irrelevant, because they weren't like this - Negan kills someone in front of Rick and says "This is your fault for going to war with us. If you hadn't attacked us, I wouldn't be killing this person right now. You got this person killed".
It's not the one death alone that he's depending on - it is the utter hopelessness Rick's group felt when they realized that there are hundreds of Saviors and they're totally ruthless. According to my count1, Rick's people had killed 43 Saviors prior to the season finale. Before the finale, Rick probably thought the Saviors were all but wiped out. When he saw them all around him in that clearing at the end of the finale, he realized he had barely made a dent in their numbers, and they were going to teach him a lesson and bring him to heel.
And think about what this means for Rick - when he was at the Hilltop, he stepped into a war against an enemy he knew nothing about. He personally rallied the troops to attack Negan. A lot of people in Alexandria will blame HIM for this. Rick looks weak, he looks like he cost someone their life in an unbelievably brutal way, and he looks like he just cost Alexandria half of its resources. That's what Negan wanted.
The message isn't "Two of your lives (Denise and the unknown victim in the finale) balances out 43 of our lives" (actually, it is now 44 lives - Morgan killed another Savior). The message is "We can (and will) do whatever we want to you, and you can't fight back or we'll murder all of you".
Of course, Negan probably expects that Rick and his group are permanently cowed and will submit to Negan forever; if you know what Rick and his group are like, however, you know that probably isn't the way it will work out.
Negan's own words:
Negan pretty much explains his MO in his speech. Negan's speech on the show was taken almost verbatim from the comics, aside from his non-stop torrent of naughty words, which were omitted from the show. I've left the naughty words in these screen caps (except for one very explicit sexual reference), but I've redacted dialogue - and omitted a number of entire panels - that would give away the identity of his victim. So if you've seen the episode where Negan is introduced, you don't have to worry about spoilers here, but if you're offended by foul language, you might want to skip this part of the post.
I've circled the dialogue bubbles containing the most important points Negan makes in red. They're not freehand circles, but this looks better.
Well, you didn't really thing you were going to get through this without being punished, now did you?
...
I don't want to kill any of you... Let me make that clear right from the get-go. I want you working for me, and you can't very well do that when you're fucking dead, now can you? I'm not growing garden.
But you killed my men. A fucking whole goddamn lot of them. More than I feel comfortable with.
For that.. you gotta fucking pay.
...
You? How stupid do you think I am? You're practically invincible. You're missing a fucking hand for fuck's sake - and you're the leader. What have these people seen you live through?
I bet they fucking worship you. I'm not going to turn you into a martyr.
The name of the game is breaking you in front of them. I'll slide my [REDACTED] down your [REDACTED] and make you thank me for it. Then they'll all fall in line.
...
It didn't have to be such a painful birth - you made it that way.
- The Walking Dead, Issue #100, Something to Fear (all emphasis in the original)
Negan's point:
He isn't trying to even the score, and he's not trying to shock the rest of the group. He's trying to show the group - especially Rick - that Rick isn't in charge any more. He's telling Rick that Negan is the boss, and Rick's reign of power is at an end. He's telling the rest of the group that the man they respect the most - who they chose to lead them - is powerless, and now answers to Negan.
Most importantly, Negan explicitly says that he wouldn't have killed anyone if Rick's group hadn't killed so many of Negan's men. If Rick hadn't gone to war with the Saviors, Negan would have either left him alone or simply said "Gimme half your stuff" without hurting anyone.
Of course, all of this is just how Negan expects it to work. Anyone who watches the show knows that Rick won't just roll over and say "that's it, we lost - he's our boss forever". And in fact, just after Negan kills [NAME REDACTED], Rick says "I'm going to kill you".
The fact that Negan doesn't retaliate for this threat probably means that Negan believes he's made his point, and everything has gone according to plan.
Would a single death phase Rick?
Yes. You mention some people Rick has lost over the course of the show. Let's take a look at them:
Herschel: Killed by the governor. Rick did everything he could to save Herschel's life, but the governor was intent on killing him.
Was it Rick's fault? Nope.
What happened next: Rick attacked the governor, and thanks to some timely intervention from Michonne, the governor died.
Was Rick affected? We don't know. He had to flee the prison with Carl, and he thought his baby had been eaten by zombies, so he was probably thinking about her, not Herschel.
Lori: Died in childbirth. The only person who might have been able to save Lori was Herschel, and he was recovering from a zombie bite and leg amputation at the time.
Was it Rick's fault? Nope.
What happened next: Rick went berserk and killed a billion zombies himself.
Was Rick affected? Absolutely. He went totally insane, had imaginary phone conversations, and started seeing ghosts.
Dale: Disemboweled by a zombie.
Was it Rick's fault? Nope.
What happened next: Not much that is relevant here.
Was Rick affected? Not much. After all, zombie attacks are routine at this point.
Sophia: Bitten and turned while lost in the woods. Rick didn't know that until zombie-Sophia staggered out of the barn.
Was it Rick's fault? Not really, but he certainly thought it was.
What happened next: When zombie-Sophia showed up, Rick took it upon himself to kill her.
Was Rick affected? Definitely, but not by her death - he was devastated by the fact that she went missing. Rick spent weeks looking for her because he blamed himself (as did Carol, for a while) for her going missing. He refused to believe that she was dead, even when everyone except Daryl had given up (even Carol).
Beth: Unintentionally shot by Dawn at the hospital. Dawn was going to let Beth go, but Beth decided to stab Dawn with a pair of scissors; Dawn inadvertently pulled the trigger by reflex, and shot Beth in the head - it was basically Beth's fault.
Was it Rick's fault? Nope.
What happened next: Daryl shot Dawn in the face.
Was Rick affected? Yes - he cried - but he didn't seem to be devastated by it, perhaps because she did it to herself.
Andrea: Bitten by a zombie while imprisoned by the governor.
Was it Rick's fault? Nope.
What happened next: Rick found her and he was sad. Andrea asked everyone except Michonne to leave the room, then shot herself.
Was Rick affected? Probably, but again, it was totally not his fault. Andrea had chosen to live with the governor.
In a fundamental way, the situation we saw in the season six midseason finale is totally unprecedented. Rick is personally responsible for getting his group into this situation, and he made all the decisions that led Negan to kill [NAME REDACTED]. He told his group everything would be okay, he led his people to war, and now someone else is paying for it.
Notes:
1The Saviors have killed 1 of Rick's people before Negan caught Rick and the others.
By this same point, Rick's group has killed:
8 Saviors on motorcycles.
18 Saviors at the armory.
7 Saviors at the slaughterhouse.
3 Saviors from Dwight's group on the railroad tracks.
4 Saviors in the pickup truck.
3 Saviors from another group, also on railroad tracks.
The number of Saviors killed by Rick's group may be slightly higher - I only included clear, incontestable deaths, but there was a lot of shooting at the armory, and there may have been some Saviors killed off-camera.
