They're not evil, and they're not good, it's not that simple. "Is it good to do a bad thing to stop a worse thing from happening, if you know the bad thing is still bad" isn't a question that has a clear yes or no answer. That moral complexity at the heart of the Halo series is one of the reasons fan like it, and react badly to works that try to say "yes the UNSC is bad" or the more rare ones that try to whitewash them.
Though as a side note I would point out asking this about the UNSC is asking the wrong question, because the UNSC didn't create the the Spartans, the Office of Naval Intelligence, a small sub branch of one service arm of the UNSC, did it. The entire US DOD isn't good or bad because the DIA did something dubious, though the higher command structure dies have some knowledge of the program (not sure how high and how much, though).
Also, some corrections:
Yeah, the whole kidnapping children and replacing them with sickly clones so their parents won't know what really happened to them is pretty evil, as is the experimentation to turn them into super-soldiers.
The clones weren't made so parents wouldn't know what happened, they were made so parents would know what happened. Having dozens of families have there kids all vanish without a trace means dozens of families frantically hunting for them trying to fund out what happened. Having dozens of families mourning their dead kid that died of some bizzare disease means they have some closure, and also don't start frantically looking for clues about what happened.
I also wouldn't call what happened "experiments", ONI knew what they were doing when the designed the process.
The fact they did it because apparently Earth can't stand the idea of the colonies breaking off and becoming self-governing just makes it that much worse.
That's not why they did it, they did it because all the data they had said that the fighting over that process would be horrifically destructive.
The pro-independence colonists were only a portion of the colonial population, and the insurrectionists were only a tiny fraction of that number, many colonies had citizens that wanted to remain in the UEG, and that if the outer colonies did break away, it would lead to a massive war between them and the UEG loyalist inner colonies.
Instead I would have done something along the lines of what IIRC was called the Spartan III program, which is to say turning people who are already adults, already in the military, and who went into it knowing full well what they were in for, and they would have come out as awesome as the game needed them to be.
In canon they considered that, the issue was that it wouldn't work without an unacceptable risk of side effects on older subjects (and given how dangerous is was for ideal candidates, that says something about how bad it would have been on adults).
spartan 3s were children specifically war orphans whose homes were destroyed by covenant, then given the option to get revenge.
Children are not mature enough to make that kind of decision, called it an "opition" is perverse. ONI took only kids that "volunteered" because it was easier to get them to cooperate, because unlike the SII's, III didn't undergo the same level of personality and psychological screening.
They were used as shock troops who took suicide missions.
Not exactly. They were used on missions that were to dangerous to risk S2s and too difficult for other special forces to accomplish. This meant that many of their missions did have an extremely high risk of them being killed, but it wasn't certain death, the UNSC took steps to extract them if they succeeded and did everything they could to ensure they would.
The second reason is they did not get as much training they were recruited during the human covenant war after all. They got basic training, but the 2s had years to master war.
That's also off. The SIII went through years of training, not quite as much as the IIs (who were largely distinguished by more experience and equipment, not more training), but the IIIs were in training for years, and were far better trained and equipped then anyone else bar the 2s.