So I have been planning a character for a long time that is a changeling rouge that is a infiltrator sort of assassin but I didn't want to use the assassin subclass (cause I generally don't like it) and the two that matched what I was wanting from the character was the mastermind mainly for there ability to very easily and quickly mimic others and basically go full shape shiftier or the soul knife because of the ability to manifest knives so they can't be confiscated and I really like most of the soul knives ability's while the mimicry thing is the only thing the mastermind has going for it which is why I'm leaning more towards soul knife.

But I was wondering what others thought about it.

Edit - please do not recommend other classes! Like how the person below did I really want to play a rouge and just wanted some advice on witch subclass would be best for my idea and how to build around it.

You might consider playing a College of Whispers Bard. Not only do they get a sub-class ability allowing them to mimic others, but Bards are a Charisma-based Expertise class (all of the relevant skills for what you want to do are Charisma skills).

nice recommendation but I've always thought of whisper bards as spreaders of misinformation and gathers of dirty secrets rather than strait up assassins so more of instigators than killers.

for example I imagine a whisper bard to inspire false fear in the masses like a walking propaganda poster, or get close to a noble so they can get black mail about that person to use later.

While I wan't my character to do some of that but instead end with the target dying instead.

You said it yourself but if there is one feature from masterminds subclass you like and you like almost all of soulknife then soulknife

Soulknife is better subclass all-around since it can give you a lot of benefits with the psionic dice and your skill proficiencies (select the right ones and you could make a great infiltrator, Skilled Origin Feat is boosted also), and all other aspects are very good in portraying what you need.

But mastermind does give you something thats difficult to get with the mimic feature (Actor Feat goes so far and the proficiencies can be obtained elsewhere). I like a lot the Master of tactics feature since it brings a big support aspect for rogues in tactical situations.

But choose the one you feel would be best for your game and the type of character you'l want.

are weapons being confiscated a real concern in your campaign or is it just a role play thing. Sleight of hand to conceal a normal dagger would cover it outside of getting fully stripped imo. but also a sling, one you can make one quick, two it is easily concealed as part of your clothing. I find psychic blades to be a trap in most campaigns. as soon as magic weapons are on the table they end up being the worse option and that is like 2/3rds of the sub class. That being said its skill boosting feature is useful. arcane trickster can cover some of this as well, change self is a level 1 spell. And thanks to true strike you can pretty easily make a int build rogue which will give a mastermind feel.