I'm so very sorry about Lazlo. Losing a dear little friend is hard enough. But when they are taken suddenly and for no apparent reason, it's just that much harder.
I just adopted my first piggies a few months ago, so haven't yet had to endure a loss. However, I have had many ratties over the years and certainly had my share of losses.
An unexpected loss really can stay with you for a long time and it can be hard to get over. It sounds like you may never know what took him, but hopefully it appeared as though he went peacefully in his hidey-house. I lost a perfectly healthy and beautiful little 4 month old rattie girl. We woke up one more to find her "gone". She had no evidence of falling, choking or anything. Her death haunted me for a long long time (still does to some extent). We just have to assume it was heart or possibly anurism related.
Lazlo would not want you to continue to fret about how he left you (I know, easier said than done)
But perhaps one day soon, you'll be able to think of all the wonderful memories of your little friend.
I hope Red continues to be healthy and happy.