So with that said, I dont think Peter has the mental fortitude
I disagree both of them have the same amount of mental fortitude. Both of them have incredible willpower. However Batman is not burdened with having a lot of immediate family not to mention Bruce has a far colder outlook on life than Pete does. Basically Peter has far more to loose than Bruce as Bruce's life is Batman. Meaning Bruce is to a certain extent more effective but he has no more mental fortitude than Parker. Mental fortitude is not a unique trait. It differs in certain situations and would depend on the context of the situation.
Plus he isn't half the detective Bruce is, so you have to double the existing body count.
Grated initially with some of Batman's smarter villains this would initially be a problem. The key word here being . Peter while not a killer does have a certain grey area as has been shown in Back In Black and Grim Hunt. If someone like the Joker did kill a bunch of people with Parker unable to stop it especially if it was people who were close to him Pete would be very likely to cripple them especially when he sees how far they go.
So yes in the short term there could be more casualties given that Parker is not used to dealing with said villains. However in the long term a lot less people would die. Due to people like Joker potentially being crippled for the rest of their miserable lives.
The eldar warlock’s eldritch fashions were crafted from pure confusion and his every step quaked with disintegrated reality. The length of the Shadowseer’s leaf-shaped witchblade sang at Ahriman, forcing the Thousand Sons sorcerer back. Ahriman barely got his Black Staff in front of the devastating sword sweeps. Immaterial energies spilled from both weapons as the psykers clashed, forcing Ahriman further back. The archway bulkhead crashed back down at the Shadowseer’s mental insistence and the metal of the Impossible Fortress’s walls melted and dribbled down across the seals, fusing the door shut and trapping the sorcerer in the pancratitaph with his enemies.
With Ahriman’s power and attention directed very much on his unwelcome visitor, Czevak’s levitating body dropped to the deck. From his perspective on the floor the inquisitor watched witchblade and force staff smash, both physically and immaterially. Where they did, the fabric of reality tore. The disciplined grace and furiosity of the Shadowseer’s assault would have carved a bloodthirster of Khorne in two. Ahriman was more than pure wanton destruction, however. The Thousand Sons sorcerer was not only an ancient and devastating warrior, he was one of the most talented psykers in the galaxy. More important than either of these facts was the torrent of unbound ambition surging through the sorcerer’s veins. He was the impossible made incarnate.
Pg. 219 Atlas Infernal
Ahriman raised a hand. The lash coiled around his arm, pulsing, cutting and sucking through his armour. Power flared from him, exploding outwards into the warp. The psychic shock wave lifted the daemons from reality