Montag remains emotionally detached in this section. He enjoys burning his own house as much as he enjoyed burning those of others, and he begins to agree with Beatty that fire is removing his problems. He imagines Mildred and his whole previous life under the ashes, and feels that he is really far away and that his body is dead. Moreover, he claims that it is not exactly he who commits Beatty’s murder—he cannot tell if it’s his hands or Beatty’s reaction to them that spurs him to the act. Beatty is described as no longer human and no longer known to Montag when he catches fire. Again, like so many other things in the novel, fire has two contradictory meanings at once. It represents Montag’s subjugation and his liberation, and he achieves his final emancipation by abusing its power. Murder is, after all, a far worse crime than book burning. Only later does Montag acknowledge what he has done and feel some remorse for his actions.

Hope this helps from sparknotes

I think he felt a little sad.