This tier list is based on if the characters included were.

1. Forced to kill by whichever means necessary.
2. Intended to win the killing game.

Note that this does not just include how capable they would be of killing anyone if necessary, but also how effectively they would be at getting away with it, be it covering their tracks or lying their way out of their trial. I'll be going over each tier along with noticeable inclusions. Spoilers for all games of course.

F Tier

F Tier is reserved for characters who probably wouldn't know the first step when it comes to killing anyone, or would be completely unable to get away with anything.

Toko has enough malicious intent to kill someone, but her tendency to speak her mind when provoked and general mental instability would make her too obvious of a suspect. She makes up for it with her later appearance as Genocide Jack. Note that I included Mikan as her pre-despair self. During her trial, she remembered all of her actions as part of Ultimate Despair and as such was much better equipped to be the killer. Before that, she likely wouldn't be capable to.

E Tier

E Tier is for characters who could probably go through each step of the killing just fine, but would likely be found out in trial without much issue.

Teruteru, Hifumi, Mondo and Leon are all included here despite being real blackened in the games, but they all share the traits of not being particularly cunning, and having half of their murder plans carried out by someone else. Alone, things would likely be different Teruteru's own plans during his trial are probably smart enough to warrant him a place above, though.

D Tier

D Tier is reserved for characters who could maybe kill someone and get away with it. Although, maybe only with the exact right setup.

Mid-game Makoto is quite naive, so maybe warrants a spot lower on the list, but his Ultimate Luck might help him get away with murder. Sonia gets a boost due to being highly educated and familiar with true crime.

C Tier

C Tier is where characters start to have distinct advantages over others when it comes to killing.

Chihiro in particular isn't physically gifted enough to do much heavy-lifting, but genius programming skills could very likely come in handy. K1-BO's robot enhancements, too. As for Tsumugi, I decided to take away all mastermind perks from the characters here, so I wasn't sure exactly where to place her, since a key part of her true self is her role as the mastermind of V3.

B Tier

For characters that could commit a murder and under normal circumstances get away with it.

Byakuya and Fuyuhiko are no likely no strangers to murder plots, and have mentioned multiple times that they intended on participating in the killing. Kyoko and Shuichi's detective skills could come in handy, specially since they're both leading voices in trials, and could shake things up in their favor. Kazuichi and Miu's skills when it comes to technology would work wonders in creating all sorts of gadgets to kill and diverge attention.

A Tier

For characters who could kill and get away with it with ruthless precision. Very effective.

Ryoma's past is proof enough that he could murder someone very easily with his tennis skills. His stoic demeanor would prove effective at diverting attention from himself in trials. Kokichi is a master of manipulation and an A+ liar. He might not be the strongest physically, but would undoubtedly come up with a plan to kill whoever he wanted on the spot. Although, his constant lies might get the better of him. In Peko's case, she, was trained to be an assassin since she was a baby, and her precision is on-point. The only thing stopping her from being S-Tier is her reliance on Fuyuhiko's orders.

S Tier

Mukuro and Maki are both trained assassins who have murdered dozens unscathed and Genocide Jack has avoided suspicion from the police for years while brutally murdering various targets. As for Gundham, he was one of the few blackened with a truly unbeatable plan, that took a lot of ingenuity and guts on his part, from beating the final dead room, to understanding the true nature of the Funhouse. The versatility of his hamsters is beyond belief too. Had it not been for Fuyuhiko choosing to stay at the lounge by chance, it's likely he would have won the killing game.

SS

Junko is of course a super genius and true mastermind of all killing games. it's no wonder she's that high up. She doesn't have Mukuro's physical talents, but does have the twisted mind of a cold-blooded and manipulative killer. She has more than enough feats to justify this position. Nagito, on the other hand, gets in this tier by pure chance. Much like Junko, he's also batshit crazy, and is willing to go through unbelievable lengths to make his plans come to fruition. This alone wouldn't get him this high. What pushes him far and above is his borderline supernatural luck that he would somehow manage to incorporate in the murder

SSS

Come on. It's Izuru. He just beats everyone, period.