It's subtle but the reason he gives in to a darker side is a mix of being tainted by the darkness and not being am entirely "good person" to begin with. The story never actually explains why he went on the original journey, but it does seem implied that he was only there because the people who were his friends asked him to be, he himself didn't care about killing the demon lord thus fueling his need for revenge for their betrayal. As for His strength gaining it never ends he is still gaining power in the last 5 chapters and these powers are often is rushed and poorly explained. I feel like this could have been a lot longer story and been better for it, But they cut straight to mostly the main plot points. Overall still a good read but not very deeply explained at any point.
So I read 4 chapters and I assume it gets better considering it's decently highly rated, however my thoughts on the first few chapters is this:
I don't understand or buy the revenge plot. The reason being that the reason his friends "betrayed" him was not only incredibly valid and clearly being proven in real time, but as the one who killed that demon king, he should be able to understand why they did it as well. Saying they should pay for it. That he wants to tear them limb from limb makes no sense to me and I simply don't buy into it at all.
Second, the training arc returning to his real strength is basically off screened. There is no chance for development in any sense of the word. No relationship development, character development, etc. They missed out on an opportunity to clearly define his character and level of strength and commitment to his cause, making it even more difficult to buy into the revenge plot.
Lastly, when the demon lord appears in his body, he had a very minimal reaction to hos friends point being proven correct and this evil being he basically gave his life to defeat being inside him. His reaction was lackluster at best. Again with the crappy revenge plot that doesn't work at all, it also just shows no character depth for him as a main character nor does it give any depth to his trials as a hero in his first life.