As told in the limited series The Umbrella Academy: Apocalypse Suite, Hargreeves was an alien from another planet disguised as a human, who after adopting seven babies all born on the same day (claiming he was doing so to save the world), disappeared for a decade. During that decade out of sight, Reginald studied the children and found that they all had superpowers. After training them in their abilities, Reginald ultimately established the superhero team known as the Umbrella Academy, with his "children" as its members. The kids fought a number of superpowered villains before ultimately splitting up due to personal differences.
Despite his goals in establishing the Umbrella Academy, Reginald did not show much love, nor give much emotional support to his children. Indeed, as shown in The Umbrella Academy: Apocalypse Suite #1 by Gerard Way and Gabriel Bá, Reginald saw his "children" as nothing more than experimental material that could be tested, prodded, and abused in order to accomplish the tasks he set out for them. This is expressed in a subtle detail about the children's designations - while they're referred to as 'Number One' and 'Number Two,' their actual titles are in the style '00.01' and '00.02.' This clinical approach shows that even in numbering the children, Hargreeves saw them as parts of a set, not as individuals. This issue also shares his notes on their abilities, describing Allison as an "insufferable, narcissistic creature, but extremely useful."
This exposes a key difference between the television series and the comic. While Reginald is clearly a "bad father" in the show, the comic reveals that there wasn't even a parental relationship. Rather the relationship was that between a scientist and his test subjects. Since it is not expected that scientists will develop "feelings" for their test material, the comic makes clear that Reginald had a similar mindset. Indeed, how could Reginald develop an essential understanding of their abilities if he was worried that they might be hurt physically or emotionally? Reginald maintained this attitude despite the fact that the children considered him to be their father, calling him "Dad" despite his instructions never to do so.
Reginald's thoughts on his children also reveal that he has no grand endgame for them. If there was, he would likely have wanted to craft them into a finely-tuned, harmonious team. To do that, however, Reginald would have needed to provide a supportive environment where they were loved for who they are, helped to develop into the best people they can be, and taught to embrace one another as family. In other words, Reginald would actually have to be a parent. Instead, as his notes in The Umbrella Academy comic show, Reginald Hargreeves only saw them as weapons of varying usefulness, and only cared about what benefits, if any, they would provide him.