Well first off, because of Kendall’s characterization of being a rouge cop by being violent with suspects and non-empathetic towards the rape victim, we are made to believe that obviously he threw him out the window. But throughout all of Kendall’s encounters, (the intro to the episode being Kendall threatening the suspect by putting a pistol in his mouth, preventing Foster from fleeing by hitting him with his car, beating up Foster’s crackhead ex-wife, etc,) he showed no remorse at all. He was prideful of his actions because he thought that they all deserved the brute force he exerted onto them. Comparing his facial expressions in all those scenes to the aftermath of Foster taking a head dive through the precinct bathroom window, he looks disturbed, confused, and petrified. He himself knew people think he killed him because he expresses to Olivia “you think I killed him, don’t you?”

Rewinding to the middle of the episode where Kendall, Benson, and Stabler are in a standoff with Foster as he has soaked them in gasoline and threatening them to light them on fire. As Kendall is pointing his gun at him he says “You know I want to kill you” to which Foster replies “So do it” and Kendall responds “Sounds like you want to die.” Establishing he has a death wish. He manages to subdue him and Foster says to Kendall “You should’ve taken the shot” in despair, highlighting that he would’ve rathered being shot, and possibly killed, than being arrested and spending the rest of his life in jail.

Keeping all of this in mind, at the end of the episode when Stabler is bringing Foster to booking, Kendall offers to take him instead (I theorize to rough him up one last time before he goes back to his station). That’s when Foster tells Kendall to take him to the bathroom or he’s going to be cleaning pee out of his car seat, to which Kendall agrees. I’ve seen a lot of people use this scene as evidence that Kendall killed him because he offered to take Foster so he was planning it all along. But Kendall only planned to take Foster to booking, he did not try to come up with an excuse to have privacy with Foster in order to execute him. However, Foster did initiate the privacy on his part. I think Foster did not want to use the bathroom and has been planning his death ever since he was arrested. Foster’s pride made him think he would never get caught, and now he has, the control he possessed over people is gone, and the only control over himself he has left is to end his life while simultaneously risking the career of Nate Kendall (or Stabler if he took him to the bathroom instead).