I answered a question second. I think the first answer is wrong - it will not misguide the OP, but it won't help either. Is it bad form to downvote said answer?
If it's wrong, or doesn't answer the question that was asked, or is misleading enough to cause trouble, or gives dangerous advice (at least without explaining how it's dangerous), downvote.
Whether or not you should downvote an answer shouldn't be affected by whether or not you have posted (or intend to post) your own answer.
Don't systematically downvote all "competing" answers! (Most usually will not deserve a downvote. Some will be good--those you should upvote!) And please don't downvote a good answer just because you don't think it's as good as your answer (or as someone else's answer). But don't let the fact that you've answered a question discourage you from downvoting another answer that merits downvoting.
Remember, proper voting--both proper upvoting and proper downvoting--are good things that contribute to the site. Please do them.