Why does Steve Rogers hate Howard and Maria Stark so much?

Except for Bucky, who actually committed the murders.

And it's up to a jury to decide if Bucky is guilty of murder or a lesser charge of manslaughter or not guilty. Certainly, Bucky choosing to flee instead of surrendering to the police shows that he feels no remorse over his actions which resulted in the violent and brutal deaths of 2 civilians so a jury may be hesitant to let someone off who shows no remorse at all for what he did.