If God would have meant what you have concluded from " I will have mercy on whom I will have mercy and compassion on whom I will have compassion", then why would he send Jesus? He can deal with sin the same way, right? He can have mercy on whom he wants to, and not on whom he doesn't want to.
Righteousness means, to give what culprit deserves, it doesn't involve contemplating on person's heart, and Mercy means to give what culprit does not deserve, which is "forgiveness."
So God cannot be Righteous and Mercy together, it is on Cross that Justice and Mercy meet, where Jesus receives the wrath of God as Justice, and we receive undeserving Mercy.
Of all the sins, biggest is to not trust the God. "Without Faith it is impossible to please the God". And faith is displayed in sufferings, not in pleasures of life. If you have Faith, you will endure suffering till the Last.
To Kill self, is to have no faith in God and not trusting God. Contemplating on it cant be sin though, as under suffering men tend to think like that, for example even Job said "It would have been good if I was killed in the womb" , and some times in Psalms too.
Of course one should not condemn those who are contemplating on suicide, but the one who is contemplating, before doing that, should think on one question "Do he/she trust God?" , and "How am I going to show my trust for God, by enduring or by ending?"