It is very difficult to present a case for anyone to kill anyone else. Very rarely have I ever thought killing justified but once in a while I think it may be. I have no idea if your image of 'Che' as a ruthless killer has any substance at all as that is not part of any history that I have read and I could not quickly find any substantiation for your claim. What I will say though is that the Batista regime was bad, bad, bad. Here is a brief extract which you can find in any history:
"
When the guerrillas took control of territory they redistributed the land amongst the peasants. In return, the peasants helped the guerrillas against Batista's soldiers. In some cases the peasants also joined Castro's army, as did students from the cities and occasionally Catholic priests.
In an effort to find out information about the rebels people were pulled in for questioning. Many innocent people were tortured. Suspects, including children, were publicly executed and then left hanging in the streets for several days as a warning to others who were considering joining the revolutionaries. The behaviour of Batista's forces increased support for the guerrillas. In 1958 forty-five organizations signed an open letter supporting the July 26 Movement. National bodies representing lawyers, architects, dentists, accountants and social workers were amongst those who signed. Castro, who had originally relied on the support of the poor, was now gaining the backing of the influential middle classes
."
Was 'Che' and others justified in executing the Batista officials who tortured innocent people and left men, women and children hanging in the street? Possibly not but I think I understand it. War is an unimaginably awful place to be and it always involves questionable executions in the field. 'Che' was just a soldier at that stage and without a shadow of a doubt he was a popular liberator which is why he is fondly remembered by many. The questionable growth of communism later, and his part in it is a different matter but his motives in fighting oppression can only be admired.
[If you have any evidence of him acting in an 'evil' way, I would really appreciate the document references. It may just be anti Cuban propaganda and there is plenty of that around.]