Using textual evidence, why does Hamlet think that killing Claudius would be an act of perfect conscience?

Hamlet believed that murdering Claudius would be justified based upon Claudius' actions.

Does it not, think'st thee, stand me now upon--
He that hath kill'd my king and whored my mother,
Popp'd in between the election and my hopes,
Thrown out his angle for my proper life,
And with such cozenage--is't not perfect conscience,
To quit him with this arm? and is't not to be damn'd,
To let this canker of our nature come
In further evil?