To understand lelouch u have to understand where he's coming from and his thoughts. I'll try to make it short.
To begin with, he saw his mother die and his sister become blind due to politically motivated actions and he spends his life in hiding for his and his sister's life. His life crumbled at a young age leading him to lose faith and trust in the world and the kindness in men's hearts. He becomes practical and analytical throwing away emotions except for his sister which he greatly cares for.

Hence a person who is practical and logical is not evil as they use methods and means that are the most efficient and effective even if others suffer. To them, it is for the greater good. Every great leader is guilty of this. He grew up as a royal surrounded by ambitious courtiers. It is vital for him to understand politics which he eventually excelled in as he understood how people thought and what to say. He manipulated others for his cause and politics is all about sweet talking and manipulation.

And even though he may seem evil, he never lost sight of his goal or never deliberately wanted to make people suffer. He wanted peace for his sister and to do that there had to be sacrifices. He knew and accepted that.
Even with Rolo, he kept saying he wanted to kill Rolo, but in the end his heart soften and he admitted his misgivings to Rolo even though he died.

But comparing Lelouch to Kira from death note as they both fall into the same circumstance and situation, Kira is evil and lelouch is not. Lelouch has virtues and morals as seen by his love of his sister and his dedication to his goal for the good of humanity. Kira has love for noone as he manipulated his family to get what he wanted betraying their love. The killing of criminals for a better world was his goal and he did it not do it just because he wanted to save the world. It was because he could do it. He had power. He was absorbed in it.