Is suicide justifiable?

Suicide is not justifiable.

However forlorn your condition may be, remember that your experiences in earth life cannot easily be obtained in spirit life. If you sever the thread of life, you take advantage of Nature's intentions and cut short your mission on earth. 

Pain sometimes teaches a lesson equally as important as that imparted by pleasure. 

The earth plane is the rudimentary stage of existence and primary lessons must be learned here.