Game Of Thrones Season 8: 11 Characters Who Could Kill Cersei Lannister
5. Euron Greyjoy

Euron Greyjoy is currently one of Cersei's closest allies, but it's still a new and relatively fragile relationship that could be brought crashing down if the pirate is capable of doing some basic maths.
As it stands, Euron thinks he's the father of Cersei's baby - the same one she was pregnant with well before she slept with him, and the one that Tyrion somehow knows about despite Euron himself only having just found out.
If Euron realises what Tyrion's words mean - and it's not a given because of how the show has portrayed him - then he could turn against Cersei. He's power hungry and a little bit mad, and he's had no issue with killing people close to him before. If he feels Cersei has betrayed him and just been using him, and realises that he can kill her to take power (or simply get revenge) then it's not out of the question he'd do it. It helps he's one of the few people who get actually get close enough to her - and do so without the Mountain being around - in order to kill her off too.