He had a heart attack. He had medical problems.
He had a heart attack, which was brought on by compression of his neck. There is a concept in the law known as the “eggshell skull” rule, which means that you are responsible for any injuries caused by your actions, even if the injured person is unusually susceptible to injury or sustains injuries that are unusually severe in relation to your actions. So, the fact that he had other medical problems doesn’t (or, rather, shouldn’t, under the law) absolve the officer of responsibility.
The Medical Examiner’s Office ruled Garner’s death a homicide caused by “compression of neck (chokehold), compression of chest and prone positioning during physical restraint by police.”
From the article.