Calling it a selfish act, I get but I don't know why people
call it absolute cowardice when death is the ultimate fear for a lot of people. There are people who didn't want to die commit that
for a cause or to save people, are they cowards too?
This one i can explain. I think this is somewhat relative to the other options available. For example, most people find sky diving terrifying; however, when skydiving is weighed against staying in a plane with a live tiger then 99% of people would jump out that plane with a smile on their face.
Jumping doesnt make them magically courageous, it only highlights their unwillingness to face the greater fear of that tiger. Suicide is very similar and would mirror the act of skydiving in that example. thus cowardice depends on the alternative.
In your example of giving up one's life to save another life; lets think about the binary option here. Effectively, Option 1= live comfortably. Option2= equals face death. In this relative comparison; the more courageous option is to face death, because most people would gladly opt to keep living comfortably. Staying alive here would be the cowardly option. This is what i mean when i say
absolute cowardice is relative to the other options available.
In most instances, suicide is indeed the absolute cowardice. Of course, i totally sympathize with when somebody chose that route due to circumstance; however it fits the definition perfectly because they are typically choosing the better option.