First, Ego loved her.
Second, if Ego needed to visit his brain to keep his human form in existence and if he didn't want her to feel bad alone, he could have simply brought her with him to Ego, the planet. BTW, she really felt bad when she was at her death bed and he wasn't there beside her (so this hypothesis is out of question).
Why exactly Ego kill her? Also, why didn't he give her instant painless death instead of making her suffer for several months?