I'm currently writing a series of stories based upon the premise that the Spartans never lost a battle since the Battle of Thermopylae so I am trying to understand their concept of war better.
If they invaded another country, would they accept surrenders or kill without mercy?
Also, this is more of a religious question, but did Spartans sacrifice human lives for offerings to their Gods?
I know it's a lot to ask, but I'd love to learn more.
I can give you some answers on this, at least according to Herodotus. The Greeks generally had some rules of war, but they were also great "innovators" when it came to waging war, so sometimes these rules went out the window. A big rule though was not destroying temples, anyone who destroyed the temple of a God would be cursed by the gods. The Athenians are a great example of this, at least according to Herodotus. When they attacked Sardis, they destroyed the temples of the Persians (and the city itself). This offended Zeus, apparently, he sent first Darius against them, and then Xerxes (who was occasional described as being Zeus at least by the Delphi Oracle) who burned Athens, and the acropolis. Gaining revenge for Sardis.
The Greeks were sometimes known to sacrifice humans, often slaves or criminals to certain gods - the Titan Chronus would have criminals sacrificed outside city gates, I've heard. But it wasn't a common or well looked up habit. But it did occaisionally happen.
I would recommend reading the first book of Thucydides, which includes the (probably fictional but highly sophisticated) arguments raised by Spartans, their allies, and their enemies, for and against starting the Peloponnesian War. It will explain a lot about notions of what justified going to war, and what other considerations were involved (costs, risks, plausible outcomes). The work is available for free through the Perseus Digital Library.
I'm not sure if your premise is meant to be an alternative history, but of course the Spartans were defeated quite frequently. After Thermopylae, the Spartans lost on Sphacteria (425 BC), at Megara (409 BC), Lechaeum (390 BC), Abydos (389 BC), Olynthus (381 BC), Tegyra (375 BC), Corcyra (373 BC), Leuctra (371 BC), Cromnus (365 BC) and Mantinea (362 BC). This is not counting severe naval defeats at Cynossema (410 BC), Cyzicus (409 BC), Arginusae (406 BC) and Cnidus (394 BC).
Greek warfare throughout this period was notoriously brutal and unrestrained, and the Spartans routinely committed acts that would be considered war crimes now (i.e. targeting civilians, killing prisoners, butchering whole populations). Only a few rules were generally observed, such as the sanctity of temples and the protection offered by events in honour of the gods (such as the Olympic Games).
And no, the Spartans did not have human sacrifice.