This is Leon we're talking about though. He's a family man, Mordred has issues with family, Leon's trusts people implicitly but he's summoning a renown traitor, Leon's not a fan of a violent first response while Mordred's first reaction to anything is 'If I throw my sword at it hard enough will this thing die?'. Leon is more along the line of servants like Diarmuid, Hassan of Cursed Arm, and Iskander. Saber aligns to people more like Kiritsugu, and Sisigou. Luckily Leon is a man of respect, and fair game which means Mordred wil at the very least respect him.