The thing is that we must understand God's grace and, in so doing, must realize that His grace is 1. far beyond our ability to comprehend, 2. is not to be used as a license to sin and 3. extends to His children who die in sin.
Now, by His children, I obviously mean those born again and living for Christ. If I tell a lie and then die instantly in a car crash, will I go to Hell or will God's mercy extend to cover that sin of lying? How limited do you believe His grace to be?
That being said, there is no reason to commit suicide. It is tragic and horrible and destructive and extremely selfish. This is the truth, however hard it is to hear it. Yet, if a Christian is pushed to their limits, see no way out, are beaten down by despair, are constantly ignored and/or attacked by those that are supposed to help them walk their Christian walk, and they, in a moment of complete loss, cry out to God and then take their own life, do we believe God would cast them away?
The danger is, of course, someone thinking that they can then end their life and go to Heaven. I am not God. I cannot say if those who kill themselves while they are covered in the blood of Christ will go to Heaven or not and since the Bible does not address this, neither can anyone else. This is the 'judgment' part of Scripture. We are not called to determine the final place of people, beit those alive or those who are gone.
So, if one thinks, "Well, I am a Christian and I just have to tell God I'm sorry while I'm dying and I'll go to Heaven," then the motives of their heart aren't pure. They are logically thinking it out and trying to kill without consequences. We must remember that God looks upon the heart.
So in that moment, when someone has made that fatal choice, someone who was truly saved, God looks at their heart in the moment and I believe that His grace is sufficient. I believe that there are some cases when suicides are forgiven and go to Heaven. But, I would not say this with absolute surety, as I am not God, as I've said, and I cannot say where someone will go when they die, even if they lived a horrendous and sinful life here. I do not know if they repented before they died, even mere moments before, and so I am not a liberty at all to say with surety where people have ended up.
This is a dangerous pitfall for believers. Some tell others that they know someone is in Heaven, and this is mostly to soothe the wounds of the person they are speaking to. Some tell others that suicides go to Hell, no matter what. But, can we honestly say? No, we cannot.