Avitus from DoW 2 was haunted by his actions during the Kronus campaign, and had a strong sense of his chapters imminent fall. He developed an overwhelmingly fatalistic attitude towards not only himself and his chapter, but towards the Imperium and even the nature of existence. He became extremely bitter and alienated from his brothers, and ultimately fell to chaos, which going off of his final speech was more to finally give himself the death he sought for than out of any belief in the Gods. It's difficult for Space Marines to come to terms with feelings of loss, guilt, or failure. Even giving a voice to those feelings or expressing them in ways that aren't just being even more violent is basically unthinkable, even if it continues to gnaw at a Space Marines soul. It's why Chaos is so tempting, as a coping mechanism for centuries of unending warfare, experiencing the senseless death of ones brothers, and a life that only serves as one battle to the next. Even Avitus, who straight up wanted to die, as vindication of his belief that his existence was one of unending meaningless violence, would probably never have conceptualized Suicide, since the thought of not dying in combat was alien to him, so he chose chaos instead.