Aye, two kids. The fall of all kingdoms. But I can appreciate the idea mentioned here. If it had been said that Logan killed them all to seize power for himself, that'd be cool. It's something that would fit him quite well, I think.
If that were true, why would he kill all of his siblings EXCEPT one? It doesn't make any sense. Also, if you read between the lines (Trollblight weapon description, character dialogue, etc.) you'll discover that Logan was a pretty decent king up until his trip to Aurora. Not only did he rid Albion of trolls (no small task), but he apparently was a very active king. He didn't just sit in his castle. Ben Finn said: "He used to go on trips all the time." Since we know that all of his soldiers who were with him were killed by The Crawler, we can deduce Logan obviously traveled with an entourage. So, while he may not have been a capital 'H' Hero, Logan was still going around exploring and killing monsters.
It's a shame that Logan's character was never developed further. He was a good man driven to terrible acts by fear and guilt. Odds are all of the people who died at The Crawlers hands were Logan's good friends.