Sure, there are other triggers, that lead him to do such a thing, which is also why I am asking, to even prevent it.
That sentence somewhat negates your previous one where you said "disease is the sole reason why he did it". No one knows why he did it, no one on this forum is any more qualified or competent to say what he was thinking or feeling than you are; nor a psychiatrist because no one can know. Does your best friend or family member know every single detail and nuance of thought and feeling you experience during the day? No. Why do you then, again, hypocritically level such demands against us, or his family?
He didn't talk to his girlfriend about how severe his illness was.
I do not know what their relationship was like. I can't comment on what he did and did not communicate to his girlfriend; and his reasons or feelings for those decisions. No one can know
find a cure
You can't cure bipolar disorder, you can medicate for it, treat it and put it into a phase of remission, but not cure it.
But there should be some indicators, some also he must have shown.
Do a small amount of homework, for all the people who have taken their lives, how many family members are left wondering why they did it? For all the people who feel depressed, or anxious, or schizophrenic and no one in their environment knows. Heck, take sexuality -- you don't know if someone is gay or not, they can hide things from you. Doesn't stop them from being gay.
And this is all I am asking, when I say, I want to understand his decision-making.
Then you have failed to read or comprehend what I wrote earlier. You cannot understand his decision-making, because you do not know him. Nor his family. His background, his history; his working environment; his emotional states and history. You can't understand something you have no conception of.
I just thought you can understand a mind alike much easier than others, who are not bipolar.
As I said before, how people experience bipolar is completely different for each individual. Read some of the posts on the forum. Some people have psychotic symptoms, others don't. What people experience in their psychoses differ. Some people experience differing severities of depression, how they experience varies -- some with existential nihilism or anhedonia; or they might have suicidal thoughts. The duration, severity and frequency of the elevated moods and what occurs in them differs too.
People from England will have similarities, but no two people in England are the same, there is a lot of variation. It's the same for bipolar.
If you want to help or know how to support, try reading a website like http://www.mind.org.uk/information-support/helping-someone-else/