He wasn't being sarcastic or perfunctory with his suggestions because he really wanted to help them. As for why he wanted to help people he wanted to kill create a better cultivation system, it's because he's always been a philanthropist.
All of his life, he has always had a vested interest in helping people and making the world a better place. Helping Wandering Thief was just one of the generous things he did to help the world. It was nothing special.
But Wandering Thief didn't take his advice for granted. This unscrupulous thief appreciated his efforts and listened attentively to everything Loki had to say as if his words were gold.
After Loki was done, Wandering Thief sighed and said, "We thought it would be easy to do. After all, you did it, and you worked alone while there are three of us."
"We thought that three heads were better than one. But only when we started did we realize we were out of our league. It made us understand just how much of a genius you are."
