Well in the future Ricky(the the time travellers are trying to kill) eventually becomes a time traveller and allegedly ruins the future. To prevent this, the time travellers are trying to go back in time and kill Ricky when he was young and defenceless. They believed they could not kill him when he is older as he would be too skilled at avoiding them since he used to be one of them. However, I believe the point of this story is that you cannot changed the future that easily. Although Ricky is unaware of his assassins purpose, he still is able to avoid them almost as thought the universe is preventing him from dying. If you would like to make a connection, try watching The Umbrella Academy. It is almost the same exact plot-line, a rouge time traveller escapes his agency and they try to kill him. In TUA, his sister is the one to ruin the future (he does not know that) and he once he find out he tries to kill her, but nevertheless the future is ruined because you cannot easily change the future. I hope this answers you question.
