The thing is, Fisk is selfish and has no qualms about getting what he wants even if literally everyone around him (some of whom he greatly respects and admires) is saying "no, don't do the thing!" The obsessive way he channeled all of his emotions into Mission: Date Vanessa basically made the whole incredibly lucrative and effective alliance of criminal organizations that he worked so hard for implode in a horrible fiery shitstorm right beneath his nose.
So what happens when the switch flips and he realizes that pursuing Matt is also something he wants, for whatever reason? Maybe he realizes that he can use a romantic/sexual entanglement to exert some sort of control over Daredevil, and that's worth way more than just killing him outright. Maybe something forced them to interact and over time he began to enjoy his company, or at least get an intoxicating rush of power from manipulating the man that almost took everything he had away from him. Maybe he just plain falls for Matt, the violent idealist whose conviction he had always admired, whose butt looks amazing in literally any pants. Maybe spending time in jail unhinged him even further and made him develop the sort of obsession he has in the comics (though preferably it does extend to a sexual interest in this story, because yum.)
Except, when Vanessa finds out she doesn't encourage him or get turned on by it or join in, she views it as the cheating as it is and basically says "no, don't do the thing!" But he's Fisk, so he does the thing anyway, and then starts flipping tables and having emotional meltdowns when doing the thing starts to ruin everything he cares about just like people warned him that it would.
Whether he knows Matt's secret identity going in or finds it out over the course of their relationship is totally up to the author.
Does Fisk feel guilty? Does he feel justified? Does he try to hide them from each other? Does Vanessa up and leave, does she try to turn a blind eye and grow progressively more dissatisfied with the situation over time or does she dig her heels in and try to force Matt back out of Fisk's life? Does she object because she has no interest in sharing Fisk, or does she do it because she's mostly worried and can see how messy and terrible it'll turn out for everyone involved and she's protective over Fisk? How do Vanessa and Matt react to each other's presence if they find themselves in a room together? Tell me, anons!
Fisk/Matt, Fisk/Vanessa, Vanessa is NOT down with Fisk cheating on her.
So what happens when the switch flips and he realizes that pursuing Matt is also something he wants, for whatever reason? Maybe he realizes that he can use a romantic/sexual entanglement to exert some sort of control over Daredevil, and that's worth way more than just killing him outright. Maybe something forced them to interact and over time he began to enjoy his company, or at least get an intoxicating rush of power from manipulating the man that almost took everything he had away from him. Maybe he just plain falls for Matt, the violent idealist whose conviction he had always admired, whose butt looks amazing in literally any pants. Maybe spending time in jail unhinged him even further and made him develop the sort of obsession he has in the comics (though preferably it does extend to a sexual interest in this story, because yum.)
Except, when Vanessa finds out she doesn't encourage him or get turned on by it or join in, she views it as the cheating as it is and basically says "no, don't do the thing!" But he's Fisk, so he does the thing anyway, and then starts flipping tables and having emotional meltdowns when doing the thing starts to ruin everything he cares about just like people warned him that it would.
Whether he knows Matt's secret identity going in or finds it out over the course of their relationship is totally up to the author.
Does Fisk feel guilty? Does he feel justified? Does he try to hide them from each other? Does Vanessa up and leave, does she try to turn a blind eye and grow progressively more dissatisfied with the situation over time or does she dig her heels in and try to force Matt back out of Fisk's life? Does she object because she has no interest in sharing Fisk, or does she do it because she's mostly worried and can see how messy and terrible it'll turn out for everyone involved and she's protective over Fisk? How do Vanessa and Matt react to each other's presence if they find themselves in a room together? Tell me, anons!