Maybe even as a back up as he had had a bad deal before and maybe he intended to confront him if it was repeated but as we have seen it did not happen like that As he passed by house he must have had some other ideas as it is not quite clear what his intentions were when going into house . Was he just trying to get there attention to get some weed and just kept going . As we know he was an adrenaline junkie . Was this just a buzz for him that led to catastrophe.If his intention when leaving the house was to get cannabis from RM, perhaps he took the knife to threaten him into giving him some.
Once he made decision to abduct Alesha there was no going back as alarm would be raised anyway and investigated so in his sick mind perhaps the only way out was to murder Alesha in a way to silence it and to hope that it would go away . From way he spoke to Mother next he obviously though he would get away with it until DNA came into frame.