In most cases Vampires have long lifespans so maybe he did it spur of the moments because he was bored or curious or plans to use the MC for some bigger agenda
He's wants to specialize in lightning because it's Heaven's strongest weapon and without it it'll be much weaker. Plus the element affinity will already give him an edge on other elements without even focusing on them.
For those dissing the MC helping people, he's doing it cause he wants to show people what they can accomplish in this game. Also, he's playing the game for fun.
I feel like he'd be a good streamer with people asking him what he's building and trying to replicate what he's building. Though I do see the fun in keeping the lead all to himself.