Actually it doesn't matter about culture whatsoever. As per the rulebooks, DnD settings which include Ravenloft are settings with Objective morality, which is not subjective - which is to say, cultural. Based on this, if a culture approves of poisons, that culture is evil, and a paladin in that culture would defy that convention and refuse it, or they would fall. Keep in mind that morality can still - and usually is - subjective for the individual in DnD. An individual can and will view certains acts as good or evil. However, their point of view does not override the overarching principle of Objective Morality, which we are playing in.
Also with the use of poisons in particular, use of poison is expressly listed as an evil act, in any situation. There was writing of exceptions to this, called Ravages - which only affected those of evil disposition, such as fiends, being of divine origin. But we are talking about poisons.
Varnishes are different to this, to a degree, by dint of the fact that they align in the category of spells such as greater magic weapon, bless weapon, deafening clang, and holy sword, which are all paladin specific and behave in a manner similar to a varnish (with the likely exception being a negative varnish, considering what it is derived from and what it is). Poisons are designed to win a fight in a dishonorable means, and are based around malice and spite as an action. Using one in such a manner, a cowardly manner overall, is an evil act for its baseness.