No it doesn't directly transfer the complete monster behavior to the killing hero but it does scare off the lesser villians who aren't complete monsters or worse forces them to step up their game to more brutal levels themselves. Also as it has been mentioned elsewhere the heroes don't have the legal authority to kill unless it is self defence. Killing in the defence of others who are not legally under your protection is a crime (at least Canada I will admit I'm not fully informed on the nuances of American law) which means they are now criminals themselves. Superheroes as criminals leaves a few options.
1) Continue to operate as they always have outside of the law but with the law actively working for them it won't last long before they are caught and depending on their power level brought down. No matter what their power level is eventually (if we ignore plot armour of course) they will be forced into one of the following options.
2) Surrender tot he authorities for their crime(s). This kind of ends their series as they are now facing life and/or death penalty.
3) Actively shut down the law. Basically the Justice Lords and the Authority's approach.
I don't see any of those options as good things.
I've never actually watched Trigun but doesn't Vash have a thou shalt not kill policy stronger than most of the DC and Marvel line up?