I understand why you are against him, because as you say he had many Muslims killed. But from what we can judge from him, Zhaahiran, his intentions were good (Allahu A`lam), so we make Du`aa that Allah Rabbul `Izzah forgives him.
I advise any person who wants to get a proper knowledge of the Shaykh to read the book "The Life of Muhammad ibn `Abdul Wahhab", written by Jamaaluddin M.Zarabozo. It is the most authentic book I have found on his life, mentioning both the good things he did and the bad; and this is surprising as the author M.Zarabozo, is a Salafi. Usually the follower will not narrate the bad things about his Imaam, like the Barelvis for example. You won't find them narrating anything bad about their "Ala Hazrat" Ahmad Reza Khan Barelvi, you'll only find exaggerations in his praise. The same goes for Hararis/Ahbaash and their Imaam `Abdullah al-Harari, and the rest of the Firaq.
But M.Zarabozo wrote this book in an unbiased manner, so it is a very good Kitaab to read to get the true picture of his life.