Why did Tomas wanted to marry my character whan the king die?
If he wanted to be the king he could have wait till the king first die and than try to kill the current heir.
Because our PC was influential; One supposes Tomas securing himself via marriage would allow him to appropriate some of our influence, which could be useful in establishing himself properly.
What @Drazen says and he is a psycho so he has a insane way to see things like a way to hurt his mother father take a revenge to been a bastard