How to make nut butter without a food processor?
There usually is no good way to do this, since most home appliances in an average kitchen are simply not suited for making nut butters. 

If you really want to do it, the preferable tool would be a masticating juicer. It is an expensive thing which also uses up quite a lot of counter space, and the process is messy. 

All other electric tools you can misuse - food processors, blenders, mills, etc. - run into the problems you already noticed, because they are not designed to make nut butter. For all, it is easy to burn them out. If you take the risk and grind away, and the motor survives, you are as likely to end up with a collection of nut pieces and nut dust as you are with a butter. And not adding oils reduces your chances of success. 

The mortar and pestle approach would work, but you will probably have to spend several hours for a jar of nut butter. It is usually not worth your time when compared to buying the nut butter. 

See also earlier questions like What features should a food processor have in order to make nut butters?, What features should I look for in a hand grinder (for peanut butter)? and How to make ground almonds creamy?.