If your opponent has an active Venom - Angelo Fortunato (-2D to non-Fist characters) and you field a Sidekick, who is knockdd out, followed by Mr. Sinister - Nathaniel Essex on level 1 or 2, who is also knocked out, thus meeting the criteria for and triggering Mr. Sinister's "ressurection" effect, does this cause an infinite loop?
From my understanding, Mr. Sinister - Nathaniel Essex will be KO'd and returned on the same level multiple times due to there being no limitation on how many times in a single turn he may be returned to the field so long as at least one of your Sidekicks were KO'd on the same turn.
There is no infinite loop. When a game effect allows you to use an ability (e.g., “If…then you may…), you may only use that ability once per game effect that triggers it. (See
viewtopic.php?f=10&t=1620). In this situation, Mr. Sinister may only be returned to the field once per time both he and a Sidekick are knocked out in the same turn. In the question you pose, only one Sidekick is knocked out, and this Mr. Sinister’s ability may only be used once.
Also please note - returning Mr. Sinister to the Field Zone isn't considered to be fielding him again.
The relevant text from the card and rulebook is as follows:
Venom – Angelo Fortunado:
While Venom is active, your opponent's non- [Fist] characters get -2A and -2D.
Mr. Sinister – Nathaniel Essex:
If Mr. Sinister and one of your Sidekicks are knocked out in the same turn, return Mr. Sinister to the field (at his same level).