I think that Osborne depicts Jimmy as having moments where he is sadistic. I don't think that he can be seen as a pure sadist because he does recognize that his love with Alison is, at its core, pure. Consider his words to her at the end of the drama: "I may be a lost cause, but I thought if you loved me, it needn't matter." For someone who is a sadist, there is a pure and unadulterated quality of seeking to inflict pain on another for its own sake. Jimmy is more complex than this. In the end, he says harsh things to Alison in the hopes of getting her to reflect true and authentic emotional states of being. There is a love that he holds for her which is present. This is not to excuse him in any way from some of the savageries her says and does to her. Yet, I am not sure he a pure sadist because of his own condition that precludes him from fully embracing anything, including sadism.
With that in mind, I think that Jimmy exhibits some sadistic qualities towards Alison. His statement to her at the end of Act I that the death of an unborn child for Alison might jolt her into what he considers to be emotional truth is sadistic. His reference to Alison as "the Lady Pusillanimous" is cruel. His lack of emotional affect to the news that Alison is pregnant is horrific. Yet, one has to see that Jimmy takes mercy on Alison when he realizes how shattered she is at having lost the baby. The notion of the old bear wandering the forest alone and his declaration of love to her is not reflective of a sadist. Had Jimmy been a sadist, he would have taken the moment of when Alison was at her lowest to exact even more pain out of her. His acknowledgement of her weakness and his frailty is what prevents him from being a sadist. He has been sadistic at moments to Alison, but he cannot be seen as a pure sadist.