it could be as mundane as depression
but then we're talking about depression and overwhelming narcissism here
because depressed suicidal people still know right and wrong: they aren't going to take 150 innocent people out with them. the desire to harm the self for various reasons is not the same as the desire to harm others. so when you're talking murder/ suicide, such as when a dad or mom kills the spouse/ kids then themselves, you're at a level far beyond and far different than just depression and suicide, you're dealing with a narcissistic asshole
if it is simply suicide and not terrorism, this suicidal guy is still a complete piece of shit on the level of a terrorist. to be so overwhelmed with such a selfish egotistical internal drama that 150 lives simply don't mean a thing? wow
man, if this is all because some fucking girl broke up with him... fuck this douchebag
It could even be
Silkair Flight 185 is particularly controversial, but the captain had rather bad money issues so on the anniversary of getting his license. He lost it big during the financial crisis, running up a debt of $1.2M.
I say "controversial" because there are those who believe it was a mechanical failure, despite the only way to reproduce the performance was deliberate inputs. The co-pilot was locked out, and both recorders were disabled (the CVR and FDR were not recording at the time of the dive). It did lead to an investigation that determined if the recorders breakers popped naturally due to a short, the recorders had enough power that they recorded the event - the CVR would record the sound of its own breaker popping. But no such sound was heard.
There was also an Egypt Air flight that was potentially caused by deliberate human inputs, again contested by the authorities involved (usually between the NTSB and the local investigation board). Of course, national pride is often at stake as well as political pressure within the country.