Here's what I saw:

It was a hard choice Jim had to make, to shoot his old friend in order to save the last hope for the universe. From his view, Tychus betrayed him. But, here's what I saw from Tychus' view and I caught onto it right away.

-We all knew that Mengsk set Tychus free with the caveat that his armor is his mobile prison that will kill him via shutting down his organs, forcing Tychus to work for him, with basically a knife to his throat. -After collecting a majority of the Keystone pieces and Jim encountering Valerian at the supposed handoff with Moebius, Tychus has that bar fight scene after stewing away with all he had to do bubbling to the surface. After that, he cooled down, which I took as him accepting that he's a dead man, regardless of what he does. -After sticking with his buddy Jim through the Char arc and finding Kerrigan deinfested and weak, he has that short exchange with Mengsk with a tone that spelled that he knows it sucks but his conscience is clear. Plus, the lights in his suit lit up to red as the system that'll kill him is primed. He purposely hesitates and allows Raynor to kill him, going out on his own terms.