Kiritsugu didn't return to his old profession, but rather he was trying to see Ilya again and his trips were him going back to the old Einzbern castle.
At the end of Fate/Zero, when he was killing the illusions of Iris and Ilya which Angra Mainyu used to speak to Kiritsugu, Angra Mainyu cursed him (what Angra Mainyu was saying as Kiritsugu was strangling it wasn't just spiteful words, it was literally cursing him)
The Type Moon Wiki describes the effects of the curse:
Angra Mainyu's curse eventually cost his life, as his limbs were lame, his sight failed, and most of his circuits became useless.
Source: Kiritsugu - Fate/Zero (Second last paragraph)
Due to the curse, he lost most of his Magecraft abilities which would allow him to pass the bounded fields that protected the old Einzbern castle.
To add insult to injury, Jubstacheit von Einzbern never allowed Kiritsugu to pass either and raised Ilya to believe she was abandoned by him. Before Ilya set off for Fuyuki, he had also ordered her to kill Kiritsugu and Emiya.
Jubstacheit is not mentioned by name, but Ilya, referring to him as grandfather, mentions being taught details about the Holy Grail War by him. He described Japanese people to her as a "neck-cutting tribe who die by having their necks cut off while sitting Japanese-style." Another mission that she is given is to slay both Kiritsugu and his adopted son, Shirou Emiya.
Source: Jubstacheit von Einzbern - Fate/Stay Night