I'm not going to argue the point of Tomoya letting Ushio overexert herself. That's pretty much given, no matter whether you believe letting her out was a disastrous decision or not (meaning whether it's really what killed her). It's not like going to a trip equals to walking all the way there by your own strength if you are sick. Even though I thought it was good to let her breath some fresh air, letting her walk until she dropped dead was highly dubious. He could have let her walk for a bit and then carried her, even if she didn't want to be carried.

Even though I said Tomoya might have realised the end is near, I'm not saying he didn't want to have Ushio keep living. But humans aren't such simple creatures. You sometimes try to do some project till the very bitter end even though you know you won't make it in time. Or you might just drop it entirely right away knowing you won't ever make it. Or you might do something the more inefficient way just for whatever vague reason.

Obviously Tomoya didn't want to shorten Ushio's life, but after watching her slowly wither for weeks, he was probably at his wit's absolute end and couldn't anymore deny her last little wish, like he said himself. And I claim at that moment he knew it would make little difference in the long run. Likewise a dude like he will still blame himself afterwards for what he did, though he did it willingly. He just carries a huge burden of regrets by nature.