First of all: I'm not asking that downvoting competing answers should be prevented by the system (there are tons of duplicates regarding this), I'm asking whether you should do it or not.
I have this silly rule that I don't down-vote answers to questions to which I posted an answer, too (competing answers). Sometimes I see horrible answers that I would normally downvote, because they're plain wrong.
The thing is that I'm not always sure if I can be really impartial, so instead I occasionally leave a comment explaining what's wrong.
Does this make sense or is this bad practice?