Asgore waits as long as he damn well can. His attacks will not kill you unless you're at 1 HP. It's made abundantly clear that he really doesn't want to kill you but was willing to throw away his life and happiness to try and give all the monsters a better future, as ill-advised as his actions were.

I seriously don't know why people depict his as some heartless barbarian. It's pretty apparent to me that if he felt there was another way (It's fairly well established that he's not the sharpest tack. Hell, someone even says that everyone knew Toriel was the brains behind the operation.), he would have taken it.

Usually people liking or not liking characters is all a matter of personal opinion and taste, but given how the entire game is trying to teach you all about kindness and forgiveness and shit, I feel like anyone so utterly despising Asgore, or any other character for that matter, for something they'd done in the past, is about as objectively wrong as they could ever be.

If you cannot forgive an Undertale character, then you have entirely missed the point of the game. You have brought great shame upon yourself, your clan, and this entire fandom. We will forgive you for that, of course, but first you will have to realize that you've done wrong and be ashamed for it.