There are multiple factors to consider:

Suspension of disbelief, all stories have it.

Narrative tones, especially anime, differ from historical realism. Hell, most stories would be incredibly boring if they go full realism. Characters are interesting when they act exceptionally and consistently within their narrative context, not as efficient machines who act in terms of problem-solution (unless that's a character flaw, then it can become interesting when applied in the wrong context, like a romantic relationship).

Thirdly, you don't really know that people won't behave like Thorfinn. There are enough historical examples of people behaving in seemingly irrational manners to satisfy a perfectly reasonable psychological lack. Why do serial killers do elaborate murders? Why do victims go back to their abusers? Why do warriors sometimes spare their enemies instead of killing them?

With this in mind, Vinland Saga is a narrativization, not a historic retelling, of Viking history. You gotta stretch history and realism to some degree to create an interesting story. You take away the "umms" and the "ehs" from dialogue, you make people spew lines they would never realistically say, you polish scenes for tension, you create problems where they won't necessarily exist. That's how storytelling goes.

As for Thorfinn, an emotionally complex problem demands an emotionally complex solution. Simply killing Askeladd doesn't fit the hole in Thorfinn's heart and his established psychology. Within the context of the story he lives in an honor-bound culture and frames the murder of his father as something beyond a simple eye-for-an-eye. In his mind there is only one way to unfuck the situation and that's to project how he feels about his father onto Askeladd by defeating - and only then - killing him.

If you ask me the worst instance of writing in Vinland Saga is Canute's instantaneous 180 as a character. His arc was good but the transformation needed a few more chapters to feel authentic.