They should have built it up more, his love for Shado seemed to have come out of nowhere. They did show Shado care for Slade when he was in very bad condition and tend to his wounds, and even get in bed with him to keep him warm. I can buy that Slade started to develop feeling for Shado then, but more than that should have been shown. It was also what he felt was betrayal on Oliver's part, that made him hate him. He spoke in season 1 about how much loyalty meant to him, after Billy Wintergreen, his partner and Godfather to his kid, betrayed him and joined Fryer's and his men.
From what he heard about what happened with Shado from Ivo, he felt Oliver had chosen Sara, an outsider who was working with Ivo, over Shado, who was part of their group and who had fought, hunted, lived and survived with Oliver and Slade on the island for almost a year, as well as look after them. He couldn't believe Oliver would chose an outsider over the people who had looked after him, survived with him and were his best and closest friends on that island. Slade was in the wrong of course, like villains usually are, but it is easy to see where he was coming from, even if more scenes with him and Shado being close should have been shown, rather than just two.
It is funny, in a way it is similar to Slade in the comics. In the comics, Slade blamed the Teen Titans for his sons death, even though H.I.V.E gave him the formula which ultimately caused Grant's death, but he still put the blame on the Titans, to complete his sons contract (in the comics, he always said that he honoured his contracts, and Slade on the show constantly saying that he keeps his promises was a reference to that). Slade put more blame on Oliver than Ivo, the same way he put more blame on H.I.V.E rather than on the Teen Titans, even though Ivo/H.I.V.E were more to blame (he obviously felt more hurt by Oliver, because Ivo was never Slade's friend, so he did not expect anything from him).
What the comics did was have it be Slade's son that caused his hatred of the heroes, and you do not have to show scenes of a father loving his son, because you just assume that they do. They did not have to build things up. Arrow chose to have it be a woman that he loved, so they had to build it up more and they didn't. That hurt the feud in a way, but Manu was still able to sell it, and Slade was still very accurate to the comics in so many ways.
