He Changed MASTERS?! | Fate/Zero Episode 9 REACTION 

You can't normally call a new servant in a Holy Grail War (but the one consistent rule of HGW is that the rules are made to be broken), so there are some exceptional circumstances Servants are also a copy of the Heroic Spirit in the Throne of Heroes, if one dies, it's just the copy that's gone and the original can be summoned by another master, another time

Lancer was implying that the reason Kayneth's fiancée wanted to become his Master and use the Grail was because she was charmed by his mole, which seems to be case given her infatuated expression

Other curiosity Lancer( Diarmuid Ua Duibhne) had a lot of problems with women in his legend. his face has a charm spell through his mole beneath his eye. Every woman who look at it falls in love with him. Diarmuid made a princess fall in love with him on the day of her wedding with Fionn mac Cumhaill and she ran off with Diarmuid. The guy who the princess were about to marry was an important guy, the leader of Knights of Fianna. The High King(the princess's father) and Fionn did not like that. They persecuted Diarmuid after that. One day Diarmuid was fatally injured, and Fionn, the only one who could have saved him(he had the ability to transform spring water in to a powerful healing agent ), refused to heal him and let him die for ruin his marriage. PS: The Three Knight Class(Saber, Archer and Lancer) has magic resistance, so Saber is not affected by Diarmuid's charm spell. Because is kinda of a low tier enchantment.

Saber just letting Lancer go last episode felt very out of character for Saber for me (I watched Fate/Zero after playing the whole Visual Novel Fate/stay night). there are a few scenes like this in Fate/Zero unfortunately