Let's also not forget that Lore not only escaped drifting in space, he also had his own personal teleportation device in his body, and who knows what other modifications he made to his body..... trying to hold him for any extended period of time against his will would have been futile, short of deactivating him.
Lore did not have a transporter built into him. Despite the bullshit presented in Nemesis such a device pretty much violates everything we know, and should know, about how the transporters work. Lore simply had a trasponder built in him to activate the transporter on his ship.
I could also point out that "technically" Data "killing" Lore wasn't murder considering, for the most part it was a "war." Between Picard's Team and Lore's team of Borg. Killing someone to prevent a galactic-wide event of murder and suffering isn't murder.
It would have been murder if, for example, someone found Hitler and but a bullet in his head. You killed a genocidist. Not a bad thing.