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More specifically, Deathrattle Viper on MM1 page 240 makes a secondary attack vs Fortitude.
From my point of view this secondary attack should not be affected by Marked condition, since it's a poisonous effect that Bite applies to the victim's wound, even if it's stated "the deathrattle viper makes a secondary attack against the same target". It seems the only reason an attack roll takes place is to decide if the target's immune system resists the poison or not.
Am I mistaken?
Yes, it affects the attack roll. You're overthinking it.
Marked opponents take a -2 to attack rolls when making attacks that don't include their marker. Deathrattle Viper's secondary attack involves an attack roll. That attack roll takes a -2 if it doesn't include the marker.
The reason for the attack roll doesn't matter, and is entirely fluff. (Seriously: the rules about powers let you rewrite the flavour text.) It could be exploding shrapnel for all it matters.
