Hi and welcome to the forum.
inject node is there only to allow you to inject messages at your will.
What I think you are wanting to do is not that difficult, it is just you haven't quite got the picture of how things work. No big deal.
Tear down services/flows nicely?
The idea is that
deploy is just that. It starts all flows.
Let's say you have a couple of flows. One is this flow of which you speak and you want it to be run .... to tear down services/flows.
And the other is another flow doing what ever.
It (the second flow) does something of importance and then requires this other event to happen: tear down services.
The last node (or close to it) will be the one which detects this thing/event/occurrence.
Connect it to the input of the first flow and that will then cause it to run.