The slap isn't so much the problem as the whole context surrounding it.
It sounds like there is far too much screen time (OK - I'm guilty of this too. We're currently trying to wean DS1 (10) down on advice of a doctor). Hence the 9yo reacts immaturely to being told he can't play on the games console. This is par for the course - he's 9, he's immature by nature - and if he's reacting in such a petty way, then I'd make a good guess either he has so much screen time he feels entitled to it at all times, or his parents never really interact with him, possibly as a result he doesn't know how to entertain himself. It almost sounds as though the screen time is only limited when one of the adults wants access to the TV - that's pretty crap, I do understand how easy it is to fall into that habit, but it's really shitty reasoning - either have screen time limits if you believe there's a max amount of reasonable screen time per day, or have a decent discussion about sharing, don't just turf the kid off when you feel like it because you trump them because you're bigger. But anyway - due to this the petty and immature reaction is only to be expected. If you're doing a concentrated effort to reduce screen time and/or emphasise that the TV is shared and others are entitled to use it too then you should be expecting a kick back as it's usual to have this reaction when you bring in any change.
That doesn't mean that it should go unchecked - but slapping isn't a reasonable response. Something like removing screen time for the next day and/or sending the child to their room (so they can't interfere with the TV) would be reasonable if they can't behave when told to come off the TV or respect that others are using it. If the TV ban is as part of changing a pattern you could also do it more gently by encouraging/suggesting other things he could do while your DH is watching TV, but I don't think that a more punitive response is harsh. Slapping is too harsh.
So there are several concerns which are basically flagging up issues with the parenting in general which might be relationship issues, if you're generally more consistent and reasonable, or might be both of you at fault, if you've slipped into lazy habits together.
First the sense of entitlement to screen time/reaction to coming off screens - may be worth a look at whether you need to reevaluate your (both of your) ideas/limits about screens. Look at your own screen use too. You can hardly blame a child for using screens incessantly if the parents also do. Again - not judging here, as it's a problem in my own family. We all use screens too much.
Second the general relationship between your DS and DH - it's clearly casual, in the sense that DS feels it's appropriate to niggle at and annoy him in the form of turning the TV on and off, but DH obviously feels that there is a hierarchy there as he can turf DS off the TV because he wants to watch it and he feels justified to use physical means to enforce this as he is bigger/the parent. So he needs to decide what the relationship is supposed to be and behave appropriately. The whole interaction almost sounds like one between brothers than father and son. In short DH needs to be aware he is a role model and act like it. How hierarchical that is is up to you (two) but it should be consistent. If DH is acting like a friend one minute and an authoritative parent the next DS is bound to be confused about where the boundary for appropriate behaviour lies.
But lastly I agree with some other posters that the response is part of what is concerning as well, because it shows that it wasn't a moment of being pushed to his limit and reacting in stress, but something he felt was appropriate and fair parenting. You don't say if you agree with smacking generally but feel that a bruise is too far, or whether you consider any smack to be violence, but again, that's something to think about.
It's not about leaving the relationship as a snap reaction to a single event, it's about looking at the event as a snapshot of your relationship and your DH's parenting (and maybe your own parenting) as a whole and deciding if you want to carry on like this, if you want to change something together - if he's even willing to work with you to change - or whether your views are actually aligned at all, and if not, one outcome of that realisation might be that you decide the relationship isn't compatible either. But even if you did end up eventually splitting up as a result of that decision, it's not in response to a single event and it's absolutely not the child's behaviour which has caused it.