Why does the fact that Claudius does not stop Gertrude from drinking the wine condemn him even more in the reader’s eyes?

Claudius could have easily stopped Gertrude from drinking the poison, but by not doing so he shows that he is a very selfish man and he doesn’t care about anyone else’s well being. It was also surprising to me though because he had married Gertrude and usually when you marry someone you have some kind of love for them, but when he let her drink the poison he is also saying that their marriage meant nothing to him. This condemns him so much more because from this we know that Claudius would kill anyone who gets in his way of being King even innocent people who haven’t done anything to him.