Suicide began to be interpreted as murder - of oneself - which is breaking one of the Commandments. For example, St Augustine, one of the early Church Fathers, wrote and had supported by his peers:

God's command ‘Thou shalt not kill,’ is to be taken as forbidding self-destruction, especially as it does not add ‘thy neighbor’, as it does when it forbids false witness, ‘Thou shalt not bear false witness against thy neighbor’ (Augustine, book I, chapter 20).

http://plato.stanford.edu/entries/suicide/#ChrPro

As an aside, it's the logic as to why it had long been considered a crime in many countries as well - in the UK, suicide was only decriminalised in 1961, until which if you failed in your attempt you could be prosecuted for attempted murder.

I can add a bit regarding the Catholic position on the off chance that the deceased or the commenters on the state of his soul are Catholic. /u/StormySan covered most of the justification, but there is some nuance in Catholicism's position.

The Catholic position on suicide has been fairly consistent, and it is laid out in an encyclical by John Paul II--Evangelium Vitae--written in 1995. We are dealing with roughly two thousand years of history here, so I will be focusing on the 20th century teaching (where I have more expertise) and avoiding much discussion of the evolution of Catholic teaching on the issue. Quoting from the Bible, Augustine, Thomas Aquinas, earlier Church documents like this text from the Vatican's doctrinal institution, as well as excerpts from Vatican II, the pontiff lays out the reasoning on the Church's position in section 66 (though reading 64-67 would give more context, and reading the entire document would give even more as many topics reinforce each other in John Paul II's writings).

Suicide is always as morally objectionable as murder. The Church's tradition has always rejected it as a gravely evil choice. Even though a certain psychological, cultural and social conditioning may induce a person to carry out an action which so radically contradicts the innate inclination to life, thus lessening or removing subjective responsibility, suicide, when viewed objectively, is a gravely immoral act. In fact, it involves the rejection of love of self and the renunciation of the obligation of justice and charity towards one's neighbour, towards the communities to which one belongs, and towards society as a whole. In its deepest reality, suicide represents a rejection of God's absolute sovereignty over life and death, as proclaimed in the prayer of the ancient sage of Israel: "You have power over life and death; you lead men down to the gates of Hades and back again" (Wis 16:13; cf. Tob 13:2)

So, while suicide is always considered a serious matter, there is a recognition that there are often circumstances that mitigate the responsibility of those who make that choice. Mental illness is given particular attention. However, suicide is still strongly discouraged as it is seen as a rejection of the gift of life and a rejection of God's control over that life.

Sacrificing one's life for a noble cause is specifically exempted from this category, as stated in the other document listed above.

However, one must clearly distinguish suicide from that sacrifice of one's life whereby for a higher cause, such as God's glory, the salvation of souls or the service of one's brethren, a person offers his or her own life or puts it in danger

For more resources, footnotes 83-87 of Evangelium Vitae would be a good starting place, but I didn't want to overwhelm you. Followup questions from OP and others are always encouraged.

A bit of misinformation going on in this thread. The fact is that there's no specific Greek/Latin word for 'suicide', so hunting for information is problematic because you have to read between the lines of the Patristic authors. Nobody explicitly mentions suicide, only certain concepts around it which together can be read as suicide. Nobody in the first 250 years of Christian literature is mentioned having committed suicide - so it's clear it's not really an issue that needs discussing (unlike abortion and infanticide), because the values about preserving life are so clear. Augustine is commonly cited as the first to denounce suicide explicitly when he simply clarified the position of previous writers (Tertullian, Clement, Justin Martyr, Chrysostom, Lactantius, Jerome), for whom suicide simply was not an option, and so didn't need debating.

I'm sorry for your loss, and would like to point out that the historical conclusions of early Christianity and your immediate issue of hell are not necessarily intertwined.