DaD!Ish's consciousness
I personally find it very interesting how the consciousness of the fully turned Ish is presented in Decayed and Decrepit. He is obviously not like the other infected. He has a faster reaction time, is able to speak and can see people from a farther distance. This is obviously connected to the fact that he's the only player-controlled infected.
But his character stands against most of the things that others (mainly DaD!Jonathan) know about the infected. Infected are normally seen as monsters who used to be human but became something else through the infection. After fully turning they are no longer considered human and no longer posses human abilities. They are controlled by the hivemind of the infection and there's nothing human left in them.
But then there is Ish. Despite being fully turned and hunting most players whenever he sees them, there are there rare moments of clarity that shine through. He led Spidey and Jonathan towards his old home to make then find the voice recording. Which infected who was completely controlled by a hivemind would do something like that when it could help Spidey and Jonathan find a way to defeat said hivemind.
I haven't really found a canon explanation for it, but instead i thought of a scenario that made sense out of canon (at least to me):
Ish knew about his infection and was aware what would happen once he fully turned. He had done enough experiments to know basically anything about the loss of humanity and that the infection takes over the body by getting rid of whatever made the person human. But he knows that he can't die yet, he knows too much that could still help some of the people who are trying to find a solution.
With that thought in mind he starts experimenting on himself in an attempt to make him somewhat resilient to the infection. Sadly it doesn't work as he'd hoped and he end up fully turning. Only that his being doesn't get taken over by the infection in its entirety. He and his thoughts are still in the body, he still sees everything the monster does. But he can't do anything. He feels the infection trying to fight against him still being there, but it can't fully erase him.
There are rare moments when Ish managed to regain control over his body. Him leading Spidey and Jonathan into the underground trap for example.
He sacrificed himself for the low chance that he's able to give the bus people enough hints for them to be able to end the infection.
Instead of choosing a merciful end, he is now stuck in a body he can rarely control himself and is forced to watch how the thing which has taken over control is trying to kill one person after another.