I don't know if evil is the right term, but there are other words that are equally effective in describing Tybalt, such as vain, extremely rude, belligerent, aggressive, and quick-tempered and prideful. He acts before he thinks and he abhors the Montagues.  All of these characteristics combine to make a character that is truly worthy of the loathing the audience feels for him, and it is due to this loathing that the audience is able to feel sympathy for Romeo’s character instead of Tybalt’s after Romeo kills him.  Shakespeare, being the talented writer that he was, knew all to well how to gain the sympathy of audience for his protagonist, and this is perhaps the reason that he made Tybalt, perhaps not evil, but very easy to dislike a great deal.