This doesn't seem to be true. According to Wikipedia, there were 2340 domestic homicides in 2007.[1] There were 18,361 homicides that year.[2] So <13% of homicides were domestic. You may have read a statistic that most women killed in homicides are killed by romantic partners - but this is not true of men, who are the overwhelming majority of homicide victims.
Even if the rate of domestic homicides doubled it would mean only a 13% increase in the overall homicide rate, far less than what we've actually observed.
> people were home more and under a lot of stress.
This explanation does not seem to fit with the social mobility data presented. The pandemic struck in March, people were staying home far more in March/April/May than in June - but homicides did not rise until June. Why?
> An increase in murder can just mean the same number of people shooting each other as normal, but more of them died.
It could mean that. But no: shootings and shooting victims increased dramatically right along with murders:
- in Chicago: https://abc7chicago.com/chicago-shootings-2020-shooting-crim...
- in NYC: https://www.wsj.com/articles/new-york-city-homicides-and-sho...
- in Portland: https://www.koin.com/news/crime/shootings-in-portland-more-t...
[1] https://en.wikipedia.org/wiki/Domestic_violence_in_the_Unite...
[2] https://www.cdc.gov/mmwr/preview/mmwrhtml/su6203a28.htm?s_ci...