<PERSON> is angry. For anyone that doesn't know the circumstances that went into why he is so angry, the anger is confused for insanity. A sufficiently angry person can act in ways that appear to be reckless abandon, and that appearance of reckless abandon could be assumed to be insanity. From the perspective of an observer, the outcome of behavior sourced either by insanity or anger is very similar. The methods may just be significantly different.
A very angry person can act in a plenty calm manner with people that haven't caused him reason to be specifically angry at them. A person may be angry (hateful even) with society at large, but be cordial and friendly with friends, family, and even strangers... until they do some specific thing that flags them as an enemy. And even then, there may not be a straight-forward attack, but some other side-band attack that is unexpected. For some, anger is a way of life that doesn't relate to mental disorder.
It is mentioned that <PERSON>, which <PERSON> sets up to die horribly, screams at him and calls him mad. In the context, it is assumed that he means that <PERSON> is insane. Lots of people say, "You're crazy." or nuts or insane. In retort, <PERSON> is not a reliable source. 494
In addition, <PERSON> is not qualified to make a medical or mental diagnosis of <PERSON>. To his credit, from <PERSON>'s perspective the distinction between <PERSON> being insane or just so angry that he's shifted to a cold, calculating, and premeditated manner is meaningless. <PERSON> knows he's going to die. It wouldn't have made any difference for him to ask "Why are you so angry?" and try to negotiate with <PERSON>. <PERSON> had decided <PERSON> was going to die, and die badly. No negotiation was going to solve it, so the distinction was (in that circumstance) meaningless. Then, setting aside that I don't credit <PERSON> with an abundance of smarts, if he thought <PERSON> was really crazy, he would have thrown out some totally illogical stuff to try to shift <PERSON>'s craziness to his benefit... possibly even letting him go. At that point, regardless whether <PERSON> was angry or insane, the gambit of "use the crazy" was <PERSON>'s only possible chance for survival, and he didn't do it.
Maybe he did earn the nickname because people thought he was crazy. He sure didn't make much effort to prove them wrong. The misdiagnosis would work in his favor because a crazy person is less of a threat than a coldblooded methodical angry person because a crazy person is simply unpredictable but an angry person is predictably lethal. (in the dwindling state of civilization he's in) Maybe when the movie was made, the ambiguity was intentional. 494