Romeo asks for forgiveness from Paris before killing himself in the tomb alongside Juliet.
The death of Romeo was tragic because he was killing himself to be with Juliet. Romeo believed that Juliet was dead because he did not receive the note explaining what was happening, and he killed himself moments before Juliet woke up from the potion.
Mercutio accepts Tybalt's challenge on Romeo's behalf. But it may be debated whether Tybalt would have murdered Romeo had Mercutio not intervened--probably not. The only point where Tybalt was actually on the point of killing Romeo was when Romeo attacked him, and Romeo was the one who prevented Tybalt from killing him.
The Friar had it planned to advise Romeo by a note, but the note never got through which led to Romeo killing himself.
Killing himself
He explains to the prince that Tybalt would have been killed anyway (for killing mercutio). And that Romeo simply just saved him the trouble by killing him himself. Romeo should have been killed for killing Tybalt, but because Benvolio reasoned this with the prince, the prince only banished him(which is being pretty nice).
Romeo nearly attempts to kill himself with a dagger before the Nurse stops him.
You just answered your own question. "Why does romeo get banished after killing Tyblat?"
Romeo Obviously. He swayed the voters but sadly he will kill himself before he realizes he actually won.. :((
It's too late to save Romeo and Juliet by then.
Romeo arrives, kills Paris and then himself.
For killing Tybalt
The Prince banishes Romeo from Verona for killing Tybalt.