I'm thinking of possibly putting an end to the start of a relationship between my ex and one of my best friend
Why do you need to stop their relationship? Don't you think you're meddling too much?
I feel like they will end up as a couple and I don't think that's the idea.
But what didn't work for you and your ex, might work out for your best friend. It's not really your choice or right to get involved with your best friends decision on this relationship. It's a hard call because if you butt in you might lose your best friend.
I'm going to go for it and do it since I'm not interested in them getting together.
If you do this, your conscience may not let you forget it and you may end up feeling very guilty by damaging a relationship just for you own good. They may be happy with each other and their unhappiness will be your doing. I do not think you want to live with that.