First off all, Tassadar was the only Protoss, who has learned to use the energies of the high- und the dark-templar together!
The second thing is, that his ship was heavily damaged by 2 Mutalisks.
And because he didn`t want more of his brethren to get killed by the Zerg he wanted to kill the Overmind on his own.
I think because of this he was a real hero!