There have been numerous theories on how Rick could turn evil
http://www.roamersan...g-bad-spoilers/
But if you don't want BB spoilers, I'll just say it
First of all, I want to say I'd be fine with Carl dying if Rick became evil as a result but he'd probably just put a gun in his mouth afterwards. Also, if Rick and Carl died, I don't think any of the supporting cast is strong enough to carry the story. I still like that one user's idea (I think it was slouhvakey) who said that Negan should win the All Out War and kill Rick. Carl runs away and is on his own for a while (it would help make him become the central part of the story), meets other survivors, becomes stronger, and eventually returns to reunite with friends and exact his revenge
Anyway, here's how Rick could become evil (and get killed by Carl later on) without Carl dying
1. A child savior kills Michonne or Andrea and Rick kills them.
2. Dwight kills Michonne or Andrea (betrays Rick) and Rick is so furious that he holds Dwight down and kills Sherry in front of him before killing Dwight
3. Maggie is going to ruin the plan (this one was likely before issue 112) and kill Negan so Rick has to kill her before she can cause the plan to fall apart
4. I have another one that I'll post on the thread I linked because it kind of has Breaking Bad spoilers so I'll be vague here. Rick realizes to kill Negan he has to become worse than Negan and ends up using Hilltop or the Kingdom as bait and causes the death of an entire community to save his own
Imagine a group of a hundred motorcycles driving down a freeway. Eventually, they hit a junction. One road goes northwest and the other goes northeast. So one guy, we'll call him S, says, "Let's go northwest!" A mile past the intersection, a semi careens into the group and kills ninety of them. Ten are wounded, but they survive and keep going. Eventually, they hit 10,000 miles. S suddenly has his consciousness thrown into his past body right before the junction. Now, he says, "Let's go northeast!" All 100 bikers survive. Happily ever after, right? But what about the ten, no nine, who went northwest and survived? What happens to the reality they were living? Does it just disappear now that S has changed the past? It's not like only bad things happened on that 10,000 mile journey. Maybe one of them fell in love with a gas station attendant and got her pregnant or maybe one adopted a homeless kid that joined the adventure. That 10,000 mile journey would be full of stories. Romances, farewells, friendships...the loss of those ninety lives is horrible and unfortunate, but what would rewriting their history mean? The nine who survived lived full lives and did the best they could with the hand they were dealt. How could it be right to just erase all that? Isn't that worth something? Is there a point to a world where everything is happy? Are people who struggle for a better life just idiots? Being human is about fighting even when it seems hopeless and finding happiness in a world that hates it. Are you saying that's worthless?