Night lords were detatched from the imperium by the means of who they recruited, and how they behaved. Iron warriors felt unappreciated and ground to the ground. Word Bearers lost the religious identity that shaped their legion and culture, so they were feeling lost without a divine purpose. Alpha Legion chose to "save" the Galaxy the way Cabal told them they can. They view themself as making a sacrificial and righteous act. Thousand sons felt opressed by the nikea edict, and backstabbed by the imperium by the space wolves attack. Death guard allegedly gave in to nurgle in order to cease the terrible torture. Emperor's children are kinda undeveloped in regards to any solid motives beyond corruption. World Eatets going bonkers trough butcher's nails into something that can hardly put thought beyond ceasing the burn of the nails by killings. Black Legion had put Horus himself above the Emperor. Some feel like they continue his will, other just see that they have no points of return. Id say you can divide the traitor legions into two branches: those who trully believe chaos to be their future and salvation, and. Those who hold personal grudges against the imperium or their vision straying from that of the emperor.
Hatred, contempt and a desire to wage war tend to be the building blocks for most traitors. Where that leads them tends to differ heavily from warband to warband, or how they vent their hunger for war in a nightmare galaxy. I'd put a longer post but, as many have pointed out, there is really no single answer here. Some are duelists hunting for a worthy opponent, some desire to venerate their gods, others to ascend to godhood, while others are madmen, victims or sadists hungering to inflict pain upon others.
I think that this link should show what "makes a Chaos Space Marine" https://1d4chan.org/wiki/What_it's_like It shows the inherent paranoia and literal Chaos in their faction, with no real way to know who's got your back and who doesn't: Should YOU stay loyal to your comrades? Or are they planning to backstab you, so you should first? Or should you do it because it'll get you more power and gear, to survive? Should you run and find new companions? Or drag along those already with you?
I must say that I wanted to post here, something simillar, yet there is no point in repeating when someone stated it better, this is the best phrasing that there could be. I totally submit to this opinion.
In order to defeat the imperium and free those under its control and censorship In addition to complete what the emperor started making mankind even better and to extinguish all xenon life. The emperor had a visio, one he rushed to which caused problems with in like morale of their soldiers and having no advancements n their technology.
