Serpent is more evil, doing Bluth's head in! (5)
I believe the answer is:
adder
'serpent' is the definition.
(adder is a kind of serpent)
I can't explain the remainder of the clue.
(Other definitions for adder that I've seen before include "Viper common in northern Eurasia" , "Mathematical reptile" , "Venomous type" , "a poisonous type" , "calculator?" .)