It is possible to kill Theo/Thea (if they have the lower relationship of the two you send to scout out Zusak in Vernex in TLH1 and you want to reconquer Tornassa, or if you botch the Thuja battle (no Belt of Speed, one unnecessarily dead friend, but at least you’re alive!) when you haven’t sent them to Zusak). However, it’s easy/intuitive enough to raise their relationship that most people don’t get them to a low enough relationship where they become the Red Warrior . Plus, I like their character, so I probably wouldn’t let them die even on an evil playthrough.
Turning a character Evil isn’t something that happens as such, but you can turn them into a vampire.
It happens later in the book not in the first one right? Anyway I wanted to know it in the case I wanted to do a Peter/Petra romance but since he/she can’t become evil I decided Jace/Jess to be my canon Love Interest
Doing that happens in the third book.
In what jobs(?) can i get specialized training?
Define specialized training. If by that you mean class levels greater than 2, pretty much any class other than Scribe, Nature Guide (although leveling Squire/Knight over level 1 is not worth it IMO because you can get the promotion for free [without having to pick it as one of your levels] in LH2, and the stat bonuses are small), or the end-game classes such as Dragon Knight, Siren, or Catalyst (which are extremely worth it) can be specialized.
If by specialized training, you mean unlocks a new class, then Nature Guide in LH1 (after finishing the Sheep job with T) unlocks Druid or Ranger if you have the stats for it (you cannot have both Druid and Ranger levels in the same playthrough). Guards can get War Master from Vale in LH2. Squire/Knight + Priest + Cleric (itself a specialization of Priest) + Good God = Paladin. Completing some of the sidequests in the beginning of LH2 will increase your versatility and even unlock new classes; for example, J’s quest gets you the Catalyst class (which is excellent thematically AND mechanically if you are romancing them) in LH3. If you have the Book of the Dead as an Evil Wizard or Priest, you can become a Necromancer. Thieves/Assassins can become Shadow Weavers if they have magic and certain items in Book 2. This is just scratching the surface of how many specialized classes there are; you can acquire them through training, through stat arrays, or even by being at the right place at the right time. The possibilities are endless!