I wouldn't claim the ability to produce the most inappropriate such message; simply a highly inappropriate one in response to that question, immediately and reflexively, whether I wanted to or not.

It is, however, superior to your suggestion for a number of reasons:

1) It's considerably more achievable to spell out a three simple-word message with no punctuation in recognisable swastikas on an easter egg than it is to spell out a five complex-word message with punctuation in recognisable heroin needles in the same medium.

2) Swastikas are an immediately recognisable and highly historically, socially and politically charged symbol in a way that needles simply are not. You'd have a hard time even getting people to recognise a pointy blob piped onto a piece of confectionery as a hypodermic needle, let alone conveying the idea that it was for the purpose of injecting heroin.

3) "Fuck the police" is also an existing politicised statement, primarily associated with black gangsta rap group N.W.A., but also a broad anarchist sentiment taken in isolation, while "Your Child's Leukemia Is Hilarious!" is simply a highly distasteful fabricated statement without any precedent impact. It makes people go "ew", unless they actually have a child with leukemia, in which case it's just grievously cruel.

4) It's composed of radically opposed concepts, instead of randomly disjoint ones. Swastikas and gangsta rap lyrics emphatically do not belong together. More generally, anarchist and fascist statements aren't the most cosy of bedfellows. Heroin needles and leukemia hilarity are just randomly thrown together. Mine is subversive in structure, whereas yours is just surreal.

5) It's generally punchier. People can interpret the components immediately, and then their brains encounter resistance as they try to put them together. You get a sudden "WTF!?" moment since it's easy to read but hard to understand. With the heroin needle leukemia hilarity suggestion, the reader is presented with two quite hard-to-interpret elements, so the confusion happens at the wrong point. It's hard to read, but easy to understand, and the payload isn't worth the cost of delivery. It's like a shaggy dog story as opposed to a snappy one-liner.

6) When someone says "Your Child's Leukemia Is Hilarious!" they're clearly just going for the most shocking and distateful ideas that they can think of, since very few people think terminal childhood illness is genuinely funny. Once you realise that, you dismiss it as kind of childish. But people do use swastikas as symbols of their allegiance to horrific ideals, and people do say "fuck the police" and mean it, because they have legitimate and complex issues with authority figures and social institutions. Taken independently they could be serious statements. Only in combination do they become a jokingly obtuse, clearly over-the-top gesture of intentional offensiveness.

Back to the broader point, I believe people with the faculty I am claiming to possess would instantly and intuitively find mine more entertaining than yours, because they will likey favour inappropriate humour.