I presume the severe damage to Saw’s body was because of the Empire, and that drives his more ruthless style of rebellion. Remember, one of the Rebel leaders actually wants him assassinated and tasks Cassian to do the deed.

Cassian killing his informant was to keep him from talking and facilitate his own escape. I don’t think he wanted to do it, but the guy was freaking out, and didn’t leave him any choice. Do we need to see regret when James Bond kills someone? It’s part of the job description.