It's very easy to judge others. Chronic depression is many times caused by something wrong with brain chemistry. That is a physical problem manifesting itself in despair and hopelessness, regardless of faith. They aren't thought processes, they are deep emotional feelings that can overwhelm someone.
Besides, it isn't up to you to judge whether someone who has committed suicide was a Christian or not, that is up to God.
1 Corinthians 2: 13 Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual.
14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
15 But he that is spiritual judgeth all things, yet he himself is judged of no man.
16 For who hath known the mind of the Lord, that he may instruct him? but we have the mind of Christ.
Lack of faith?:
“Blessed are the poor in spirit, for theirs is the kingdom of heaven.”