Tybalt is portrayed as a villian in the play, but why is his death so bad for Romeo

Well ... Tybalt is Juliet's cousin. Although he is mischievous and a bit of a rogue, he is still well-loved by Juliet and the rest of her family (the entire Caulets).

Romeo is now a murderer (forcing him into a rapid departure), and a murderer of a member of Juliet's family to boot! If things weren't already against him in blood alone, this act would certainly sour any advancements he could have possibly made in asking Juliet's father for her had in marriage.