Abstaining from voting makes you somewhat complicit in whoever wins. You have the ability to affect the outcome with whatever choice you make (Harris, Trump, neither). If you choose neither, it is partially your fault the winner won as you could have voted against them.

It can be boiled down to a classic trolley problem. A greater harm the trolley is hurling towards, a lesser harm you could divert the trolley to. You can choose inaction and let the greater harm happen or you can choose action and cause the lesser harm. Most people think the lesser harm, even if they enact it, is better. But it's a classic morality problem for a reason. Some people view the action to cause the lesser harm as less moral even if it prevents the greater harm.