The solution is not simple. It's not just guns, its not just mental health. There are so many layers to this issue.
Taking the approach that we can't stop everyone so we shouldn't do anything is foolish, why have any laws?
You're absolutely right that its impossible to stop someone who wants to cause harm, but should we not even impede them? Don't you think every one of those staff and students would have preferred him to have a hand gun, knife, or even a homemade explosive? If I had to take my chances, I'll take them with those over an AR-15.
That said, simply banning guns isn't the answer and quite frankly I haven't heard too many people even propose that. Tackling these issues will require efforts in multiple areas perhaps starting with gun control and mental health but that alone will not be enough. People need to learn to pay attention to the signs and speak up. Law enforcement agencies need to handle complaints better than they have in this situation, where maybe this could have been prevented.
As someone who attended Douglas and grew up in Parkland, I'm not going to sit back and watch politicians sit back and do nothing. I don't have the answers but I know maintaining the status quo is not going to work. It's not the time to rush to extreme solutions, either, but we cannot sit idly by and do nothing. We all need to stop with the partisan bullshit on both sides for a little bit and work on solutions that make us safe. Both the left and the right will need to give up a little to work for the common good.
Simply stating that guns are not part of the problem is as foolish as saying the only problem is guns. Let's accept that the answer never lies in the extreme but somewhere in the middle.