Consecutive 11 to 12 hour, multi-sector duty days are not uncommon in this business so why would AC need 3 crew for the single leg, Toronto to London?
Illness, in any shape or form is liable to occur at any time in any given percentage of the population. So why are people surprised when it occurs to a pilot? I know we are treated like tireless machines but we are just as human as the next man. Screening or whatever you want to call it is only as good as the last assessment session and hence, virtually pointless.
Stresses build up within people for any number of reasons and we all handle them differently. This poor guy apparently had enough stress to tip him over his personal control threshold. His inability to deal with it in this instance should not condemn him to be pilloried by the media and certainly not by us here on pprune. Leave him alone; next time it might be you.
I hope he gets better soon. With proper treatment and perhaps some appropriate medication, I trust he will.
Dr Rubik