Answer:
Claudius is confessing to have killed his own brother.
Step-by-step explanation:
The excerpt presented in the question above is an excerpt from "Hamlet," a play written by Shakespeare. In this play, King Hamlet, is assassinated by Claudius, his biological brother who usurps the throne, becoming the new king. The plot continues with the life of Prince Hamlet, who tries to find out who murdered his father, so that he can take revenge.
Even before this revenge takes place, Claudius is already suffering psychologically for the murder he committed, realizing that this made him a most horrendous sinner, as you can see in the excerpt presented above, where he confesses that he killed King Hamlet.