Why do criminals care SO MUCH about "grasses" and a strong desire not to be imprisoned despite...

... them being obvious criminals and knowing that the law at least exists in a systematic fashion, why does it look down on grasses to the point of having them hunted down and KILLED (isn't murder like THE worst offense, especially one like this where the person is innocent or poses no real threat etc) for it, can't they just, even if it makes them angry, accept it as a general rule even if they don't agree to it etc?

And if this is the case, couldn't "grasses" like our character here be made somewhat ANONYMOUS and be given full protection and just let them go?