the THING about lucimort is that i think voldemort against his better judgement fell for some of the stuff lucius said. at least a little. lucius is a fast talker he's a sweet talker he's a flatterer. and everyone who knows him SHOULD know better but they don't!
and he doesn't even lie. he DOES believe that voldemort is the best wizard ever and he DOES intend to be forever loyal. and he's the kind of guy who will be forever loyal and do whatever you say so long as you keep a tight grip on him, stay in power, and give him the power he wants. and voldemort never believed he wouldn't be! so they're really where they want to be.
and Lucius is competent and good at getting things done but also a dog willing to suffer for his position. so he's the one who ends up with the diary and dumbledore calls him "lord voldemort's favorite death eater"
but lucius (forever the scorpion) throws away his diary and denies him to the ministry (refuses to go to azkaban for him!) and really what could you expect...
and then of course severus sells him out and he's not even useful anymore, but voldemort can't bring himself to get rid of him. he can just target the things lucius loves more than him (and power, and the death eaters) - his family