Why did Hashem kill Pharaoh's horses in the yam suf? What did the horses do wrong to deserve to die? Isn't that tzar baleie chayim?
According to the Mechilta (Tractate Shirata, 2) the horses did indeed do something wrong:
The Holy One, ever to be blessed, would bring the horse and his rider and make them stand trial. God would say to the horse: Why did you run after My children? The horse would answer: The Egyptian drove me against my will, as it is said: “And the Egyptians pursued,” etc. (Ex. 14.9). God would then say to the Egyptian: Why did you pursue My children? And he would answer: It was the horse that ran away with me against my will, as it is said: “For the horses of Pharaoh went in,” etc. (Ex. 15.19). What would God do? God would make the man ride upon the horse and thus judge them together, as it is said: “The horse and his rider hath God thrown into the sea.”
According to Rambam (Moreh Nevuchim 3:17):
והוא שאני אאמין שההשגחה האלהית אמנם היא בזה העולם התחתון – רצוני לומר: מתחת גלגל הירח – באישי מין האדם לבד, וזה המין לבדו הוא אשר כל עניני אישיו ומה שישיגם מטוב או רע נמשך אחר הדין, כמו שאמר ״כי כל דרכיו משפט״; אבל שאר בעלי החיים וכל שכן הצמחים וזולתם, דעתי בהם דעת אריסטו, לא אאמין כלל שזה העלה נפל בהשגחה בו, ולא שזה העכביש טרף זה הזבוב בגזרה מאת האלוה ורצונו האישי עתה , ולא שהרוק אשר רקק אותו ראובן התנועע עד שנפל על זה היתוש במקום מיוחד והרגו בגזרת האלוה ולא שזה הדג חטף ובלע זאת התולעת מעל פני המים ברצון אלהי אישי – אבל אלה כולם אצלי במקרה גמור כמו שחושב אריסטו.
... It is this: In the lower or sublunary portion of the Universe Divine Providence does not extend to the individual members of species except in the case of mankind. It is only in this species that the incidents in the existence of the individual beings, their good and evil fortunes, are the result of justice, in accordance with the words, “For all His ways are judgment.” But I agree with Aristotle as regards all other living beings, and à fortiori as regards plants and all the rest of earthly creatures. For I do not believe that it is through the interference of Divine Providence that a certain leaf drops [from a tree], nor do I hold that when a certain spider catches a certain fly, that this is the direct result of a special decree and will of God in that moment; it is not by a particular Divine decree that the spittle of a certain person moved, fell on a certain gnat in a certain place, and killed it; nor is it by the direct will of God that a certain fish catches and swallows a certain worm on the surface of the water. In all these cases the action is, according to my opinion, entirely due to chance, as taught by Aristotle.
So I would say that per the Rambam, there is no particular reason why the horses were killed, but as there would not have been a reason to do a miracle to save them.
If they were saved it would have been by one of two ways. Either Kriyas Yam Suf would have been a smaller nes or there would have been a special nes to save the horses. Klal Yisroel was supposed to have the nes of Kriyas Nes so the former was not going to happen.
That leaves the latter option. Aside for the chance that those horses would then quickly turn into idols, the skeptics would claim that it was the horses who made the nes, the nes wasn’t made for kllal yisroel because the horses were also saved through a nes etc.
Let’s also ask the question the other way around, what did these horses ever do that they were more deserving of a nes than any other horses in history whose owners led them on dangerous paths?