Surely he realized that the best way to get the Baudelaires to cooperate in his play when the time came was to be as nice to them as possible. Had he given each of them their own room with a cozy bed, not made them do horrible chores, not abused them, and gave them well-nourishing meals, the Baudelaires wouldn't have suspected a thing when he asked them to be in his play. They would have gladly participated and he would have gotten their fortune without any resistance. But because of how he did treat them, the Baudelaires knew that there was no way he was just inviting them into his play to be welcoming.
Now I know that some people reading this might think that it's difficult for someone who's so consumed in greed and hatred to act nice. However, Count Olaf did manage to act nice in later books. Whenever he was in one of disguises, he acted polite to the adults he was trying to fool and he also acted polite to the Baudelaires in front of those adults.
Considering his exchange with the hook-handed man at the end of the dinner party (TBB p. 54), it seems clear that he hadn't thought of the marriage and play plot at first, and was still mulling over ways of getting the Baudelaire fortune and whether it would even be possible. It seems to me that his first thought was simply to use the Baudelaires in whatever way satisfied his grudge, and it was only once he came up with a workable plan which required their co-operation - and Mr. Poe called him up to inform him that the Baudelaires had complained, which is perhaps more agency from them than he had expected - that he made more of an effort.
It's shown numerous times that Olaf, while for the most part is rather ignorant and simple minded with many things, he does have a knack for creating some of his evil plans.
Of course, his house was a wreck. It would have just been easier to tie up someone and use thier home to pass off as his own. Rather than attempt to fix it up and clean. But that's also the point of the evil characters in the whole series, they never are truly "nice" to the Baudelaires. Hey always have an off putting approach to them. So he could have attempted to be nice, but nothing about Olaf is nice. Even when he tries.
Dr. Orwell is conspicuously nice to the Baudelaires, explicitly citing a "you catch more flies with honey than with vinegar" approach, but this lasts for all of a couple of pages before we understand that she is an accomplice of Count Olaf. Esmé, by contrast, doesn't really bother to be anything but herself.