Instead of a simply putting them in an ambush battle that will ultimately end in a t.p.k., is to use other 'avenues of attack'.

##Examples:##

You can have the wizard ambush them and finish them quickly, but leave them alive, having 'learned their lesson'. All damage can subdue (on the last hit) instead of kill in 5e, so you don't have to kill them outright to prove a point. You could even have him attempt to recruit them while they are near dead, or have them locked up in a torture chamber of some sort.

You can have the wizard convince (through magic or money) the local law enforcement to detain the player characters for looting through his home. This has the potential of making the wizard look more resourceful and dangerous because he has some of the 'good guys' on his side.

You can decide he doesn't want to risk a close quarters combat when he wasn't expecting one, so he runs off, but will come after them later.

##The Point:##

Any of these things leaves him to be dealt with later, and turns him in to an ongoing antagonist, able to influence more of the storyline. There are an infinite number of ways of dealing with the problem that don't result in a T.P.K. yet can still fit a villain, depending on their mindset.

If you want him to be flat out lethal, than do so. It is not necessarily wrong to do that. If you are questioning whether it is too brutal to do so, however, then it comes across as you not wanting to do this and want to see if a story-appropriate alternative can be found. Think of the character's motivation and personality, and base the response on that.

If you do decide to have him set up an ambush, drop hints that he is flat out dangerous... Something for the players to see that reflect his capability such as material components for the fireball spell in a small pouch on the ground or in his home and notes about the damage it can do (in in-character terms, not dice) or something similar.

Instead of a simply putting them in an ambush battle that will ultimately end in a t.p.k., is to use other 'avenues of attack'.

Examples:

You can have the wizard ambush them and finish them quickly, but leave them alive, having 'learned their lesson'. All damage can subdue (on the last hit) instead of kill in 5e, so you don't have to kill them outright to prove a point. You could even have him attempt to recruit them while they are near dead, or have them locked up in a torture chamber of some sort.

You can have the wizard convince (through magic or money) the local law enforcement to detain the player characters for looting through his home. This has the potential of making the wizard look more resourceful and dangerous because he has some of the 'good guys' on his side.

You can decide he doesn't want to risk a close quarters combat when he wasn't expecting one, so he runs off, but will come after them later.

The Point:

Any of these things leaves him to be dealt with later, and turns him in to an ongoing antagonist, able to influence more of the storyline. There are an infinite number of ways of dealing with the problem that don't result in a T.P.K. yet can still fit a villain, depending on their mindset.

If you want him to be flat out lethal, than do so. It is not necessarily wrong to do that. If you are questioning whether it is too brutal to do so, however, then it comes across as you not wanting to do this and want to see if a story-appropriate alternative can be found. Think of the character's motivation and personality, and base the response on that.

If you do decide to have him set up an ambush, drop hints that he is flat out dangerous... Something for the players to see that reflect his capability such as material components for the fireball spell in a small pouch on the ground or in his home and notes about the damage it can do (in in-character terms, not dice) or something similar.