The fact that thanos is looking loki right in the eye as he puts the power stone to Thor's head like. "Come on idiot I know ur full of shit and don't want me to do this"
Thanos: I'm going to kill your brother, you good with that?
Thanos: *Starts to kill Thor*
Loki: --NOT. ABSOLUTELY NOT. YOU DIDN'T LET ME FINISH
i know you're trying to make a joke, but like. Bro. I could go on for hours and hours about this scene and the implications of Loki's relationship with Thanos and break down as many details as humanly possible, but no one has asked that, so instead I'm just going to say this:
Thanos was completely aware that Loki was full of crap and just wanted to do it anyway
Thanos looked at loki and grinned as he tortured Thor, completely aware of the emotional bond between the two of them. He just wanted to do it anyway.
Loki didn't look at Thanos the entire time he looked at Thor. The only time that Loki looks away from Thanos in his scenes in IW is when he looks at Thor, but this is the first time that Loki looks away from Thanos in the scene and it shows a shift in his priorities -- stopping Thanos from hurting him to Thor. Thanos watched this and kept doing it anyway because he wanted to.
Behind Loki, the Black Order is grinning/smiling. This is a familiar scene to them: Loki begging Thanos to stop hurting him and being unable to. Makes you wonder how much of the torture Thanos inflicted was public and/or with the help of the Black Order. Loki barely seems aware of them at all, like this is something he's dealt with forever.
Thanos could have beaten it out of Loki. He could have grabbed any random Asgardian and it would have been just as effective. Thanos singled out Thor and he tortured Thor because he wanted to and he wanted to see Loki crumple.
Thanos just wanted to do this.