The Farmer and the Viper/Playing With

Basic Trope: Always Chaotic Evil creature or individual shows no gratitude for good deeds performed for it.
 * Straight: Farmer Bob takes in a baby orc and nurses it to health. It attempts to murder his daughter, Alice, so it can have all the attention.
 * Exaggerated: Farmer Bob takes in a baby orc and nurses it to health. It promptly rips him in half, massacres his family, and burns down his house.
 * Justified: The orc operates on instinct, and simply doesn't know any better
 * Inverted: Orcs take in and raise a human child, but the child refuses to stoop to their levels of barbarism.
 * Farmer Bob takes in a baby orc and nurses it to health. Then Bob goes mad and attempts to murder his own daughter Alice, but the orc stops him.
 * Subverted: The baby orc lunges at Alice, and it seems like he's attacking her... but he was actually knocking her away from a scorpion, which he promptly slays.
 * Double Subverted: At least, that's his excuse when he's caught; the claimed scorpion is nowhere to be found.
 * Parodied: The baby orc claws at farmer Bob's face, screams bloody murder, and goes on to attempt increasingly obvious and outlandish ways to kill him and his family, but farmer Bob writes it off as as "spirited behavior".
 * Deconstructed: Orcs aren't Always Chaotic Evil, Bob just brought in a nasty one, but with his daughter dead, refuses to believe that, and rounds up an Angry Mob to wipe out all orc-kind.
 * Reconstructed: The baby orc's act was actually just the tip of the iceberg that begins to reveal orcs really are Always Chaotic Evil, just good at hiding it when its convenient.
 * Zig Zagged: An unkempt, careworn drifter comes to your house. Since you're Genre Savvy, you'll let him in, give him something to eat and a place to sleep for the night. Maybe he'll be an Angel Unaware. It turns out that, nope, he's a con man, and you find his bed, and the safe where you kept your valuables, empty. Damn. So, a few months later, another drifter comes by, asking for a place to stay for the night. Since you're now poor due to that last bastard, you refuse him room and board. It later turns out that this was a Secret Test of Character and you just failed. Sigh. Damned if you do, damned if you don't...
 * Averted: Farmer Bob takes in a baby orc and nurses it to health. It grows up to be a relatively stable child.
 * Enforced: Writer Tim wants to have the orc child be nice, but it's a medieval fantasy, and he has to stick to the standard that makes all orcs barbaric monsters.
 * Lampshaded: "So you took in an orphan child from a race known for their wicked ways and never considered it might relapse?"
 * Invoked: Orcs leave their young out in the wastes in the hopes well-meaning fools will take them in.
 * Defied: The young orc struggles with his instinctive wickedness, but his better nature wins out in the end.
 * Farmer Bob knows that even young orcs are dangerous, ungrateful beasts, and leaves it to starve.
 * Discussed: "Orcs are dangerous, no matter what kindness you show them."
 * Conversed: "Wait, so even the youngest and most helpless of orcs are irredeemably evil? What the hell?

Excuse me, could you please give me a hand back to The Farmer and The Viper? STAB.