I guess I can see how it would have fucked everything up to tell Luke the truth at that time. You can see Alex Guinness’ eyes flicker right before he tells the lie, btw. You can almost see him thinking “Fuck me, if I tell this little bitch all the goddamn hell his motherfucking father raised… damn, he’ll flip his shit and think of nothing except how to run his little ass as far from the Force as possible. Hell with it.l Imma lie.”