Guys, he deactivated his own brother, that was cold. Also I feel Lore had so much misspent potential. He was clearly as intelligent as Data but had some form of ADHD which stopped him from realizing it. He was a nice person, just a bit screwed up, if Soong had programmed him better he could have been a mischievous pranskter instead of a deranged psychobot.
Ok first of all, Lore had no form of ADHD...
1.) ADHD is a human condition (Lore is an Android)
2.) Lore remained completely in control of himself (he displayed no traits of hyperactivity, his attention didn't ever sway away from the task at hand, except when he was pretending that he was limited in knowledge during "Datalore")
3.) Lore was not deranged or psycho, rather his values and ideals were extreme to the point where he believed he was better than everyone else (Superiority Complex) which was further highlighted in "Descent Parts 1 and 2"
4.) Data deactivated Lore because he was too dangerous to be left active, even in a confined environment, someone with Data's level of intelligence and resourcefulness could eventually find a way to escape, Lore did indeed have the level of intellect that Data had, perhaps even more...(after all Lore had the benefit of Emotional Intelligence from his initial activation) (Lore for instance discovered a way to communicate with the Crystalline Entity, something which took Data several years to accomplish and even then it was only theoretical until he combined his research with Dr Marr)
5) In relation to Data's percieved "Coldness" - Data has no emotions, he has only his ethical subroutines, logic, the cultural values of the Federation and his friends/colleagues and his Starfleet experience to guide him in making decisions, he has no emotional gravities when making decisions. He does things almost always by the book (which led La Forge and Wesley to come to the conclusion that Data was still alive in "The Most Toys")