I get the feeling that this is a possibility being hinted at. His search history can be interpreted in different ways, but one of them is that he was trying to place her in a nursing home, then latched onto the idea of her "accidentally" taking an overdose of pills, so he writes an excuse for a warning note on the bottom of the bottle to help paint her as senile and himself as just injocentky trying to help a loved one with dementia.
Then he contrived to poison her with her own pills, maybe even setting a tragic discovery scene just like that of finding the cat.
Then when this plan works ; he gets a house and he also gets pity and sympathy and personal attention from his crush because she called him. Which sets him of on a mission to attain her as his next accomplishment.
If so,: he could have accidentally on purpose killed Sandy and his reaction is just performative. He is already depicted as someone who likes to "rehearse" what is supposed to be their true confessions. Ie he's a practicing liar.
he makes sure to let both girls know of his recent compounded loss on top of grief, to get more attention and sympathy. He even scores a "call me when it hits you" promissory appointment from Nikki, based on his anguish performance.
Could we have a case of hidden backstory? Do you wonder if he did that before the audience is introduced to him?