The alternate question is "Did the devil force Karras to jump out the window?" This is a subject that depends on how the audience interprets the argument: in a traditional cinematic way or a theological explanation.
Theory 1: Viewers who have seen the movie generally agree that it was Karras' intent to jump from the window. He dragged the devil from Regan's body into his own, but then his hands headed for Regan's throat. At that split second, he did what he felt he had to do, which was leap to certain death. This is supported visually in the movie: when the devil takes possession of Karras, his irises turn the demonic yellow that we had seen in Regan; an internal struggle ensues, which Karras appears to win - his irises return to normal, and he THEN utters the "No!" and throws himself from the window. Rather than call it "suicide," which would contradict the director's intention of Karras' renewed faith, viewers tend to think of it more like "demonicide" (note: Roman Catholic theology that previously stated the souls of those who commit suicide were doomed to hell was changed during the Second Vatican Council due to a better and more modern understanding of mental illness. The film takes place after the reforms of the Council was enacted). Karras rediscovers his faith at the moment he has the confidence to say, "You son of a bitch!" Clearly, he's no longer talking to Regan, but to the demon inside her. Until then, Karras as a psychiatrist could have thought he was dealing with a rare form of mental illness combined with paranormal phenomena. But when the demon mocks the death of Fr. Merrin, something 'clicks' inside of Karras, and he sees the demon's - and God's - reality in one flash. He wins the battle by giving up his own life as opposed to letting the devil take Reagan's life. In other words, Karras did not commit suicide, no more than did Jesus in his self-sacrifice on the cross or a soldier who throws himself on a grenade to save his comrades.
Theory 2: This scene is meant to be a parallel with an exorcism found in the Bible (Mark 5:1-20), where Jesus drags a "Legion" of demons out of a man. Then, the "Legion" begged to Jesus so they could possess any body instead of that man's, so Jesus allowed them to enter the bodies of 2000 pigs (unclean animals according to the Kosher rules), but the pigs then ran off of a cliff, to die by drowning in the sea. Note that Father Karras repeatedly says he's a faithless man guided by science, giving him a sort-of "unclean" status before God. He's symbolically speaking as a "pig" willingly to receive a devil as a desperate act of obedience to God. Given this, when the devil goes out of Regan, it enters in Karras, who in the same fashion of the pigs in the biblical account, (a) prefers to die atoning for his own disbelief rather to let a devil enter inside to destroy him or any other person, or (b) the devil indeed wanted to destroy life, regardless of "clean" or "unclean." Some knowledge of the Bible would be required to substantiate this interpretation, and thus it seems less likely than Theory 1.
Theory 3: Karras takes the demon from Regan, expels it, but is thrown out of Regan's window during the struggle. Karras's eyes turn color, like Regan's, to indicate the demon is inside him but just before he goes through the window they change back to their normal color, leaving us to wonder if the demon left his body just as quickly as the demon came into it. Seemingly Karras's intention of throwing himself out the window was to bring the devil with him to his death, but perhaps he was too late and the devil went back into Regan or into thin air and seemingly, somewhere, into another person. Perhaps Karras's eyes turn back to normal to indicate Karras has the power to temporarily suppress the devil inside him and thus is successful at killing the devil. Also, Karras is still barely alive as Father Dyer comes to him on the sidewalk, the demon seemingly gone from his body. Could the demon have passed into Dyer? During the process, Karras seems to float effortlessly in the air, something he would be incapable of doing on his own so it could be construed the devil is taking Karras's life, having no intention of remaining inside Karras. This interpretation, however, is invalidated by Blatty's sequel, Legion (filmed as Exorcist III: Legion), which shows Karras in a mental institution years later, still very much alive, and still possessed by the demon (as well as by at least one more persona, an executed serial killer).