The reason Catholics hope their loved ones haven't commited suicide is because it is highly possible that a person who does so will not escape Hell. Of course, no earthly being knows for sure the final disposition of a soul. However, if a person commits a mortal sin without repentence, Catholic theology states that person goes to Hell. A mortal sin is constituted by three parts: Grave Matter (taking one's life would be one); Full Knowledge (one is aware that this is a grave sin); and Deliberate Consent (fully and freely determines to do so.)
Certainly, not all people who commit suicide would be engaging in all three of these. Nevertheless, Catholics are taught that suicide is extremely serious, with potentially infinite, disastrous, effects on the soul. This is quite a simplification, as there are many, many other aspects related to such things, such as how it affects the souls of other people, as well as the person's soul who does it.
With regard to "I would like to think my loved one had made their own decision rather than being brutally killed by another", that is contrary to the Catholic position, which is that people aren't free to make such decisions. We are created by God, and are subjects of Him -it's simply not within our purview to decide such things. Moreover, Catholics look at all suffering as an opportunity to join our suffering to His and strive to, with His Grace, be grateful for the opportunity to do so. It's sort of the antithesis of the contemporary culture view (suffering is bad, therefore to be avoided at all costs.) Suffering is really at the very basis of Christianity, because it was though suffering that we have the opportunity for everlasting life! (Our Lord consents to die on the Cross.)
People have very different world views. To Catholics, we hope our loved ones haven't despaired of God's mercy, which is what many people who commit suicide do.