Of the people who were killed while on a shooting spree, is there any prior evidence that they were in fact suffering from a mental illness?

Of the people who were killed? I doubt it, they were kids.

gun-control argument
I guess, in Canada, you cannot have a sensible argument.

I guess, are there signs? Probably?
Is this a public health issue?
No, Crazy violent people are going to be crazy, violent.

You need to look at rates of crime in sensible areas, like here, to unpack the statistics.
When crazy violent people go crazy, when they have access to automatic weapons, there will be mass murder,
When crazy violent people go crazy, when they have not access to automatic weapons, there will probably not be mass murder.

It isn't an issue of crazy.