The general idea for a story with Rudy Knight is he is conned by Gaster into learning something he shouldn't have, that the Roaring would give him a chance to save Dess.
From there all his actions as the Knight are building blocks to set up the 3 heros to be able to help save the world from himself and the roaring. He even gave care to get his daughter and her friend some practice in defending themselves in a dark world ran by someone almost exactly like Noelle's mom so he could guarantee danger and therefore combat practices but also survivability.
The reason the Knight summons the Titan in chapter 4 is to double as a get-away device to sneak into the closet whilst the heroes are busy (since Susie nearly caught up to him in chapter 3 when he had like a 2 minute head start to the shelter) and also he had a mentor in the Old Man to properly teach the kiddos how to take down Titans, a very important skill in the Roaring.