Can I choose to not use the Ranger's Colossus Slayer feature when I hit an injured target with a bow and arrow?
When my DM found out about Colossus Slayer, he started making all my 1s on attack rolls a guaranteed hit against someone in my party as long as they're in my range. We're level 3 at the moment, and essentially half- and one-shotting my teammates every time I roll a 1 feels pretty sucky.
Best Answer
As far as the rules are concerned, no.
There's no "you can" or anything like that here, it's given as a definitive statement. When you hit a creature with a weapon attack, it takes extra damage. So if you don't want to deal extra damage, don't hit it with a weapon attack.
With that said, ask your DM - they might be totally ok with you choosing not to gain the benefit of one of your class features. After all, if you didn't have the ability, there wouldn't even be a question.
With that also said, it sounds like you're worried about killing something. It's worth pointing out here that as long as you use a melee attack, you can knock an enemy out rather than killing them if you reduce their hit points to 0.