It seems to me that Trump became very afraid and acted in panic.
As you know, Comey was leading investigation about Russian influence on US presidential elections and their close ties with Trump's team.
Do you remember last US American president who did the same thing?
It was President Nixon.
He tried to stop investigation about himself but because of great public pressure, he couldn't stop it and eventually he resigned.
In my opinion, the same thing will happen to Trump.
He can't be above the law.