Is it better to be left on read or un-read? I guess it depends on if you know that they saw it.
If it's impossible for them not to have seen it, then it's better to be left on read. In that case, being left on read is an acknowledgement of your message. Like, "Hey dude, I saw this. But I just didn't get a chance to reply. Or I'm just too tired to reply now but just letting you know—I saw it."
Or, it could be worse because now you know they're ignoring you and couldn't care less about replying, nor about how their behavior would make you feel.
Though... if you've been left un-read, it kind of means that they couldn't care enough to even open the message. Or they cared so much about ignoring you that they marked the message un-read after opening it so you couldn't be sure they had seen it. This sense of uncertainty reeks worse than knowing. It's the effort of hiding their acknowledgement of your message that makes un-read worse than read.
Or.
It could make un-read better than on read, since un-read may mean that they're taking the time to make you not feel bad that they saw it and didn't reply. Unlike before, now it's the effort of hiding their disregard of your presence with the intent of preventing you from feeling bad that they ignored your presence. It all boils down to the intent of marking you un-read.
Or.
It could be that they're taking time to draft a perfect response (the ideal situation—though kind of not. I'd rather they just reply). Like you do with your emails. You see it one second, draft a preliminary response in your mind, mark it un-read, and move on. And then forget about the message until a few days later. And you start your "perfected" reply with the adulterating "So sorry for the delayed response...."
And so it goes.
Or.
It could just be that they didn't see it.
I must postface that this overthinking is quite unnecessary and the whole shenanigan* is contingent upon your relationship with the person on the other side of the screen.
*Shenanigan: I looked up if shenanigan was an actual term, since I've only ever heard the plural form, but I needed to use the singular in this case. And yes, shenanigan does come in the singular as well.
SO TRUE!!!