There is always someone who pops up on these threads and says "Oh but MN just hates sms" - as if, for some inexplicable reason, they just decide to hate for the sake of hating and as an automatic or kneejerk response, when, in fact, most of the times sms get a hard time on these threads is when they are showcasing the very issues that people consider problematic.
I think what MN does feel quite strongly about, however, is the fact that parenting is a joy but also - and more so - a responsibility. And with responsibility comes lots of reasons that as we parents we can't just put our wants first because we, well ... want to. Bringing a third party into a dc's life and home is a huge deal to them in terms of impact, and it needs to be approached with the dc's needs forward and centre.
And while a SM will never replace a parent, in walking into a long term relationship with a partner who has children, I believe they also, as decent human beings, have a responsibility to be honest with themselves as to whether they are prepared for the dc's needs to occupy that space in the new family arrangements, and, simply, whether they are really cut out for the role of SP. Hence the irritation of many MNers when someone walks into that role then comes on here complaining about the foreseeable as if it were a particularly nasty trick the kid has played on them.
I don't think it is a remotely easy role.
I know sms who have literally (and I use that word in its proper sense) been saviours for their scs, so they aren't just "bad guys" because of the step title. But sadly that isn't the norm - and, I think, for lots of understandable reasons. As @SnowSolst commented "I can no longer do other people's kids" - and I don't think that is an unusual reaction. DC are annoying, they are demanding, they are a style-cramping burden as well as a (very) great joy. In your situation, there is the additional complication of the dc being badly parented, which only compounds all those things. There is no shame in not wanting to be up for that. But I do think there is shame in not wanting to be up for that but also wanting to press on regardless of the fact you know you aren't. I do think that if you feel you resent this child, you have your answer as to whether it would be responsible of you to press ahead with this relationship.