None of your options are pretty here–where you say “evil tendencies inhibited” what you’re saying is that it is either mind-controlling them, or subverting their personality. Most of these options are very evil…but may not be illegal since your politician is subjecting a “monster” to them. But, as I say, you do have options. We’ll start with things such as spells/items.
WARNING: Make sure your players are okay with this sort of mental subversion–what your politician is doing is extremely abusive behavior.
5th level spell. Depending on how powerful of a caster laid it down, it would need to be renewed occasionally.
You place a magical command on a creature that you can see within range, forcing it to carry out some service or refrain from some action or course of activity as you decide. If the creature can understand you, it must succeed on a Wisdom saving throw or become charmed by you for the duration. While the creature is charmed by you, it takes 5d10 psychic damage each time it acts in a manner directly counter to your instructions, but no more than once each day.
In short, you give the Lamia a fairly broad-reaching Geas, such as “Be a good wife” or “Never do something that would harm me in any way” and it will take massive psychic damage if it disobeys.
It’d take a while and multiple repeat castings of this spell, but a high enough level spellcaster can, given time, rewrite sufficient chunks of an individual’s memory. Do this enough, and you can rescript their personality.
While this charm lasts, you can affect the target’s memory of an event that it experienced within the last 24 hours and that lasted no more than 10 minutes. You can permanently eliminate all memory of the event, allow the target to recall the event with perfect clarity and exacting detail, change its memory of the details of the event, or create a memory of some other event.
At Higher Levels. If you cast this spell using a spell slot of 6th level or higher, you can alter the target’s memories of an event that took place up to 7 days ago (6th level), 30 days ago (7th level), 1 year ago (8th level), or any time in the creature’s past (9th level).
Remove Curse or Greater Restoration purges all instances of this spell.
You blast the mind of a creature that you can see within range, attempting to shatter its intellect and personality.
8th level spell so this is on the rather powerful end, but a sufficiently powerful Feeblemind is unrecoverable (Int of 1 gives a -5 to saves, with no proficiency bonus, a DC of 16 is insurmountable). A feebleminded individual is basically incapable of taking initiative on anything.
This is more about changing alignment, but this is an optional rule/plane from the DMG where anyone taking a long rest on that plane has to make a Wisdom Save or become Lawful or Neutral Good–keep em there 1d4 days and it becomes a permanent change.
Caution: Send a patsy to do this with a lie about why you want it done…if your politician goes, he might risk, gasp, becoming a decent person.
As DM, you can make up magic items however you’d like. Perhaps there’s a collar that can cast Dominate Monster on the creature who is wearing it, controlled by the person who is attuned to it. Snake-girl behaves herself out of fear of the collar being activated. Or, perhaps, Reverse Sanctuary…it’s a cursed item that makes its wearer act like everyone else is protected by the Sanctuary spell.
The alignment specified in a monster’s stat block is the default. Feel free to depart from it and change a monster’s alignment to suit the needs of your campaign. If you want a good-aligned green dragon or an evil storm giant, there’s nothing stopping you.
If it suits the needs of your story for this NPC to be non-evil, then go ahead and make that change.
This is going deep into “Your players might be uncomfortable with this” territory…but you don’t need magic to change something’s behavior. Abusers do it to their victims IRL on a regular basis. Punish ‘undesired’ behavior, reward ‘desired’ behavior, and force a dependency on you.
If you still want to include magic in this, then might I suggest giving a read to this reddit post (too long to transcribe here) that provides a breakdown of how a spellcaster used relatively low-level magic to shatter the sanity of an unsuspecting girl. Obvious warning on that link, as it talks through some serious abuse.
You don’t have to explain every minute detail of a setting to your players. Your Politician has a snake girl that would normally be a dangerous monster as a pet. He probably did something unsavory to attain this control, but if he doesn’t explain what he did–then the PCs will never learn the details.