I'm not convinced. Hasegawa says he'll "make her disappear", which is a pretty vague statement that imo seems to imply kidnapping more than dismemberment. After all, she doesn't actually disappear in the end. These cases have all had a twist to them and I think the author still has something up his sleeve. Note also that none of the actions shown here have anything to do with the mystery person putting on gloves near Godai's apartment.
Current idea is it's a police higher-up using the stalker situation as a cover to murder the father and prevent his report from becoming public. You kill the whole family to mask the true target. As for the dismemberment idk, maybe he's just a sadist (or perhaps a torture method to get the info?)