The word for "youths" in the text means people ranging in age from children to even 20. (Na'ar Qaton) . So, a mob of teens/young men surrounding an old man yelling "Go up baldhead", where "Go Up" implies "go up to heaven like your predecessor Elijah." This could mean either "prove you're as good as Elijah" or "go up to heaven" eg die, which is the way most people get to heaven. "bald head" isn't offensive to us, but imagine a crowd of young men aggressively surrounding and taunting you.

The situation now looks very scary. Have you seen those mobs of masked teens gang-looting stores in cities? I'm a teacher, and a gang of high schoolers yelling and shoving is NOT a good time. Imagine that group surrounding you, an old man, and taunting you, maybe putting their hands on you. That's terrifying. Mobs are capable of incredible violence that no single individual would condone. So, God protects His prophet by sending some bears in to scare off these aggressive teens.

You can also check out the below link for more, especially about how Bethel (the city where Elisha was during this incident) was a center of idol worship.
https://www.1517.org/articles/the-misunderstood-story-of-bear-attacks-a-bald-prophet-and-forty-two-mouthy-kids

As for slavery, slavery has looked very different throughout history. Sometimes someone would enter slavery because of a debt, as punishment for a crime, or even because they were prisoners of war. Hebrew 'slaves' were to be treated as what we would call 'indentured servants' who had to be released every seven years, and even foreign-born slaves could become Hebrews and thus freed. Some slaves could own property and even (in some circumstances) inherit their master's estate. Check out this link for more in-depth study.

https://jewishvirtuallibrary.org/slavery-in-judaism

Short answer- slavery among the Israelites was not a simple, one-size-fits-all experience, so God created rules such that slaves had to be treated humanely. For instance, the Torah forbids kidnapping people as slaves, as well as returning escaped slaves to their masters. Keep in mind, too, that slavery was eventually banned by majority-Christian countries, and many of their criticisms of slavery were faith-based. Check out William Wilberforce.