In Attack of the Clones, Jango Fett kills Zam with a dart while Obi Wan holds her and Anakin interrogates her. Why wouldn't Jango Fett kill Obi Wan and Anakin, too? Wouldn't this be the logical thing to do for a cold blooded bounty hunter like Jango to end inevitable further investigation?
It can actually be seen as part of the whole plot to destroy the Republic. If Jango had killed Obi-Wan Kenobi, then Obi-Wan would never have found Kamino and the clones, which was a trigger point for introducing them into the war and the ultimate instigation of Order 66.
Therefore the plan to destroy the Jedi and Republic from within would have failed/never come into action.
There is, to my recollection, no canon answer as to why jango did not kill Obi-Wan and Anakin (i.e. there is no material where Jango explicilty says why), but given the circumstances, a few assumptions can be made.
1. He wasn't paid to.
Jango Fett was hired to kill Padmé Amidala, not the two jedi. When his subcontractor (Zam Wessell) failed to do the deed, he decided to off her to remove all traces leading back to him. That was simply cleanup, not business.
Killing two (or one for that matter) jedi would likely result in a, so to speak, crusade for his capture. The jedi wold likely go out in force to find the culprit. Hard to do business when a legion of space wizards are gunning for you.
As far as I know there isn't anything in canon which explains this, but one possibility nobody's mentioned yet is that Jango could only fire one shot before alerting the Jedi to his presence. After the first shot he lost the element of surprise. If he had used that first shot to kill Obi-Wan or Anakin instead of Zam, the remaining Jedi would likely have succeeded in interrogating her and learning Jango's identity. By silencing Zam instead, he succeeded in keeping his identity a secret, for the moment at least.
As for why he didn't try to kill Obi-Wan and Anakin in addition to Zam, without concern for the element of surprise, that's because he had no reason to think he could take on two Jedi and win. You'll note that later in the film even when faced with Obi-Wan, a single Jedi, on Kamino he tries to run rather than fight.
One Problem that was not mentioned so far is also that the dart itself was especially prepared by the Kaminosians....to kill Zam. It was specially crafted just for her.
So the dart wouldn't work for a Jedi. And if he had tried to blast the Jedi.....they have a very good reputation to not be very suspencible for surprise attacks (except if they trust someone or the plot aehem...the force calls for it).
Thus not only would killing a Jedi be a kill he wouldn't be paid for and get hunted for by Jedi out of all things.....it wasn't an assured kill like Zams. And killing Zam would also eliminate all needs to kill the Jedi as there was nothing pointing towards him (and Zams death was already planed anyway as the dart was prepared way ahead of time already).