About a month ago (Skyrim time) I was hanging out in Riverwood when I was approached by a man named Sven. He explained about how he was in love with a woman named Camilla, but that a Wood Elf named Faendal has been trying to court her too. Sven asked me to make Faendal look bad, but being the upstanding snitch that I am, I told Camilla about Sven's little plan.
About a week goes by, Camilla and I fell for each other, got married, and moved into my house in Whiterun. A few days later I returned from adventuring to see a Wood Elf leaving my house, but I think nothing of it at the time and went about my selling, crafting, etc. When I returned from my next adventure I find a Wood Elf in my house, chatting it up with my wife. It's that *******
Faendal! In my *******
that! I thought, Next chance I get...
Anyway, A day goes by and I come up with the perfect plan. I saddled up and south to Riverwood in search of Faendal. I told him I need his help hunting for some leather for my Iron Dagger buisness back home. He agreed to come along and we road till we were at a giant camp west of Whiterun.
When we arrived, there were no giants in sight, but three very large, very cranky mammoths. Then I asked Faendal to prove his loyalty to me by attacking the mammoths. Guilt stricken, he said a prayer, pulled out his bow, and obliged. Moments later he was thrown to the ground by a mammoth. Seeing Faendal fall to his knees like that, making his last promises to whatever heathen god those elves worship, something came over me. I ran headlong into the fray, charged up and released my soul trap spell. In one powerful swing of my sword, I filled my first black soul gem with that bastard Faendal's soul, and sent what was left of him to Oblivion.
I left his body there, laying in the rain, trampled by the mammoths. It was a long walk home that night. Nobody saw a thing, no one even saw me leave Riverwood with him. I had done it, I had gotten away with murder. I felt terrible.
Even though we never spoke of it, my wife has never looked at me the same way after that. I could tell she knew just by looking in her eye.
I still have that black soul gem. It now sits in a bowl on my nightstand as a chilling reminder of the monster that now lives inside me.
TL;DR: Got married then caught some wood elf trying to hit on my wife. I took him into the wilderness and absorbed his soul, ended his life and made it look like a hunting accident.