Though some cannot find it in their hearts to forgive what they deem to be unforgivable, it is not quite the same as "dancing" on their graves....
And anyway, no amount of praying for , OR damning of the departed will sway final Judgement either way.
God is not confined to the parameters of time, as we are.
A prayer or sacrifice for someone now will be known to an eternal God when that person for whom the prayer is offered comes before Him for judgement.
It is a holy and wholesome thought to pray for the dead. Your prayer for a suicide could be the intercession that caused a last second conversion, or that influenced the eternal God to grant salvation. After all, no greater love does one have than the love one shows in giving his life, his love, his prayers, for his fellow man.