You should always seek to vote based on the content of the answer, not by the current score positive or negative. That's what your vote is counting for, after all - vote for what you think, not for what other people think. Their own votes are used for their opinions, after all.
The sum total of the votes on a post is meant to express a sum total of people's opinion of the post. So if a post has one downvote and one upvote, it should indicate that one person found the question useful, and one person found it not useful. If that upvote was meant solely to balance the downvote, then it is deceiving people on two ends - it's lying that someone found the post useful, and it's lying that the voter found the post useful.
If you must vote up a user out of pity, at least make sure it's a post that you find useful enough to upvote in the first place. Pity may be a turning point if you're undecided about voting, but it shouldn't be the sole motivation.