So let's say this was a confirmed terrorist attack (it isn't confirmed, and I personally think it's too amateur and low damage to be a targeted terror attack). In theory, that would help Trump's message.
But by saying what he just said, he now gives Hillary a chance to latch on and point out how horribly unprepared he is, and that he didn't put the victims first, he put his own campaign first. It's very easy to take what he said and build a narrative around it that all he cares about is his campaign and maybe he was actually hoping the damage was worse than it was.
Clinton wouldn't and shouldn't say all that. Just say her heart goes out to the victims with injury. Then prod at Trump for jumping to conclusions without all the facts first.