I think it's a bit of a myth that Dayna keeps missing chances to kill Servalan.
In Powerplay, Volcano, Moloch, Games, Orbit and Warlord they're never in the same room together IIRC. Rumours of Death and Sand as well, thinking about it (though both are near misses, and in Rumours the implication is that they've made sure Dayna isn't there when Avon goes to question Servalan, just in case).
In Harvest of Kairos and Terminal Dayna's disarmed and effectively a prisoner when they do meet.
In Deathwatch, there's a clear plot reason why Avon tells her "Don't kill Servalan." Same point pretty much holds for Gold though it's less clearly stated. In Assassin, I think, Dayna grumbles about how "I almost had her," when she's teleported out as she's about to fire. And in Traitor she only just gets a glimpse of Servalan for a moment and doesn't quite believe it till Tarrant agrees that she wasn't seeing things (seeing as until then they'd both thought she'd died with Liberator).
So that really only leaves Animals and maybe Children of Auron (isn't that another disarmed situation?) to grumble about.
Well in Animals she's brainwashed so we can let her off I guess...
She definitely has opportunity in Aftermath, although Avon tells her not to because he needs Orac it’s not like she has to do what he tells her.
Maybe she doesn’t get that many clear cut opportunities, but she has more reason to want Servalan dead than anyone, which means that when she has a shot and doesn’t take it it’s far more obvious, and she can’t play the rational card that Avon, Tarrant, Villa or even Cally could.
I know Avon specifically instructs her not to kill Servalan in Death Watch, but she does have a gun to her head and it must at least cross her mind that she may never get another opportunity.
As for the debacle that is her failure to kill Servalan in Assassin, that just goes to show that, despite her many talents, Simon rarely convincingly sells the fact that Dayna is supposed to be a deadly warrior.
And in Terminal why does she just teleport down with one gun? This is a woman who’s a genius with weaponry who has, on several occasions, pulled bizarre weapons out of thin air, yet she doesn’t even think to slip something up her sleeve? Especially when she must halfway suspect she’s going to her death?
I know it’s the nature of drama, it’s just harder to suspend your disbelief when someone has that much of a grudge.
I guess the only explanation is that, for all her deadliness, Dayna lacks the killer instinct that Soolin has.
I think if Soolin had been on Helotrix Sleer would have had a smoking hole in her forehead
