Im sure some of it has to do with Saudis wanting to fuck Russia over Syria
It has to do with a weak demand and an abundant production.
Usage is increasing slower than supply is increasing.
Oil has a large markup due to scarcity created from cartelling, and that markup is contracting.
At a lower price, usage will increase more swiftly, until then we should expect the price to over-correct.
As for renewable energy being more expensive? That depends somewhat on what is included in the cost.
I agree with you on oil. As for "renewable" energy, it ends up being more costly than natural resources.